diff --git a/.gitignore b/.gitignore
index 0853b5ab2..d85815f18 100644
--- a/.gitignore
+++ b/.gitignore
@@ -31,6 +31,7 @@ package-lock.json
 .node-version
 # When installing software on gitpod.io, `core.*` files are generated
 /core.*
+*.swp
 
 # For 0x flamegraph directory (https://github.com/davidmarkclements/0x)
 *.0x/
diff --git a/changelog_unreleased/flow/14619.md b/changelog_unreleased/flow/14619.md
new file mode 100644
index 000000000..224afe2d4
--- /dev/null
+++ b/changelog_unreleased/flow/14619.md
@@ -0,0 +1,12 @@
+#### Support Mapped Types and keyof (#14619 by @jbrown215)
+
+```jsx
+// Input
+type Mapped = {[key in keyof O]:number};
+
+// Prettier stable
+// Does not parse
+
+// Prettier main
+type Mapped = { [key in keyof O]: number };
+```
diff --git a/cspell.json b/cspell.json
index 2a58e552c..4a0d8afd1 100644
--- a/cspell.json
+++ b/cspell.json
@@ -326,6 +326,7 @@
         "tldr",
         "Tomasek",
         "toplevel",
+        "Tparam",
         "Tradeshift",
         "Transloadit",
         "trippable",
diff --git a/src/language-js/print/flow.js b/src/language-js/print/flow.js
index 1cade1bb5..93f0550c1 100644
--- a/src/language-js/print/flow.js
+++ b/src/language-js/print/flow.js
@@ -44,6 +44,7 @@ import {
   printDeclareToken,
 } from "./misc.js";
 import { printTernary } from "./ternary.js";
+import { printFlowMappedTypeProperty } from "./mapped-type.js";
 
 function printFlow(path, options, print) {
   const { node } = path;
@@ -178,9 +179,12 @@ function printFlow(path, options, print) {
       assert.ok(kind === "plus" || kind === "minus");
       return kind === "plus" ? "+" : "-";
     }
+    case "KeyofTypeAnnotation":
+      return ["keyof ", print("argument")];
     case "ObjectTypeCallProperty":
       return [node.static ? "static " : "", print("value")];
-
+    case "ObjectTypeMappedTypeProperty":
+      return printFlowMappedTypeProperty(path, options, print);
     case "ObjectTypeIndexer":
       return [
         node.static ? "static " : "",
diff --git a/src/language-js/print/mapped-type.js b/src/language-js/print/mapped-type.js
new file mode 100644
index 000000000..d8411b7d1
--- /dev/null
+++ b/src/language-js/print/mapped-type.js
@@ -0,0 +1,88 @@
+import { printDanglingComments } from "../../main/comments/print.js";
+import hasNewlineInRange from "../../utils/has-newline-in-range.js";
+import { locStart, locEnd } from "../loc.js";
+import {
+  group,
+  softline,
+  indent,
+  ifBreak,
+  line,
+} from "../../document/builders.js";
+
+/**
+ * @param {string | null} optional
+ * @returns {string}
+ */
+function printFlowMappedTypeOptionalModifier(optional) {
+  switch (optional) {
+    case null:
+      return "";
+    case "PlusOptional":
+      return "+?";
+    case "MinusOptional":
+      return "-?";
+    case "Optional":
+      return "?";
+  }
+}
+
+function printFlowMappedTypeProperty(path, options, print) {
+  const { node } = path;
+  return group([
+    node.variance ? print("variance") : "",
+    "[",
+    indent([print("keyTparam"), " in ", print("sourceType")]),
+    "]",
+    printFlowMappedTypeOptionalModifier(node.optional),
+    ": ",
+    print("propType"),
+  ]);
+}
+
+/**
+ * @param {string} tokenNode
+ * @param {string} keyword
+ * @returns {string}
+ */
+function printTypeScriptMappedTypeModifier(tokenNode, keyword) {
+  if (tokenNode === "+" || tokenNode === "-") {
+    return tokenNode + keyword;
+  }
+
+  return keyword;
+}
+
+function printTypescriptMappedType(path, options, print) {
+  const { node } = path;
+  const shouldBreak = hasNewlineInRange(
+    options.originalText,
+    locStart(node),
+    locEnd(node)
+  );
+
+  return group(
+    [
+      "{",
+      indent([
+        options.bracketSpacing ? line : softline,
+        print("typeParameter"),
+        node.optional
+          ? printTypeScriptMappedTypeModifier(node.optional, "?")
+          : "",
+        node.typeAnnotation ? ": " : "",
+        print("typeAnnotation"),
+        options.semi ? ifBreak(";") : "",
+      ]),
+      printDanglingComments(path, options),
+      options.bracketSpacing ? line : softline,
+      "}",
+    ],
+    { shouldBreak }
+  );
+}
+
+export {
+  printFlowMappedTypeProperty,
+  printTypeScriptMappedTypeModifier,
+  printTypescriptMappedType,
+};
diff --git a/src/language-js/print/type-parameters.js b/src/language-js/print/type-parameters.js
index 998e44eb3..eea098dd2 100644
--- a/src/language-js/print/type-parameters.js
+++ b/src/language-js/print/type-parameters.js
@@ -16,7 +16,6 @@ import {
   shouldPrintComma,
   getFunctionParameters,
   isObjectType,
-  getTypeScriptMappedTypeModifier,
 } from "../utils/index.js";
 import createGroupIdMapper from "../../utils/create-group-id-mapper.js";
 import {
@@ -24,6 +23,7 @@ import {
   shouldHugType,
 } from "./type-annotation.js";
 import { isArrowFunctionVariableDeclarator } from "./assignment.js";
+import { printTypeScriptMappedTypeModifier } from "./mapped-type.js";
 
 const getTypeParametersGroupId = createGroupIdMapper("typeParameters");
 
@@ -119,7 +119,7 @@ function printTypeParameter(path, options, print) {
   if (parent.type === "TSMappedType") {
     if (parent.readonly) {
       parts.push(
-        getTypeScriptMappedTypeModifier(parent.readonly, "readonly"),
+        printTypeScriptMappedTypeModifier(parent.readonly, "readonly"),
         " "
       );
     }
diff --git a/src/language-js/print/typescript.js b/src/language-js/print/typescript.js
index 5565ec850..0f5367df1 100644
--- a/src/language-js/print/typescript.js
+++ b/src/language-js/print/typescript.js
@@ -1,8 +1,5 @@
-import { printDanglingComments } from "../../main/comments/print.js";
-import hasNewlineInRange from "../../utils/has-newline-in-range.js";
 import {
   join,
-  line,
   softline,
   group,
   indent,
@@ -12,7 +9,6 @@ import {
 import UnexpectedNodeError from "../../utils/unexpected-node-error.js";
 import {
   isStringLiteral,
-  getTypeScriptMappedTypeModifier,
   shouldPrintComma,
   isCallExpression,
   isMemberExpression,
@@ -20,7 +16,7 @@ import {
   isObjectOrRecordExpression,
 } from "../utils/index.js";
 import isTsKeywordType from "../utils/is-ts-keyword-type.js";
-import { locStart, locEnd } from "../loc.js";
+import { locStart } from "../loc.js";
 
 import {
   printOptionalToken,
@@ -58,6 +54,7 @@ import {
 } from "./type-annotation.js";
 import { printEnumDeclaration, printEnumMember } from "./enum.js";
 import { printImportKind } from "./module.js";
+import { printTypescriptMappedType } from "./mapped-type.js";
 
 function printTypescript(path, options, print) {
   const { node } = path;
@@ -239,32 +236,10 @@ function printTypescript(path, options, print) {
 
     case "TSTypeOperator":
       return [node.operator, " ", print("typeAnnotation")];
-    case "TSMappedType": {
-      const shouldBreak = hasNewlineInRange(
-        options.originalText,
-        locStart(node),
-        locEnd(node)
-      );
-      return group(
-        [
-          "{",
-          indent([
-            options.bracketSpacing ? line : softline,
-            print("typeParameter"),
-            node.optional
-              ? getTypeScriptMappedTypeModifier(node.optional, "?")
-              : "",
-            node.typeAnnotation ? ": " : "",
-            print("typeAnnotation"),
-            ifBreak(semi),
-          ]),
-          printDanglingComments(path, options),
-          options.bracketSpacing ? line : softline,
-          "}",
-        ],
-        { shouldBreak }
-      );
-    }
+
+    case "TSMappedType":
+      return printTypescriptMappedType(path, options, print);
+
     case "TSMethodSignature": {
       const kind = node.kind && node.kind !== "method" ? `${node.kind} ` : "";
       parts.push(
diff --git a/src/language-js/traverse/visitor-keys.evaluate.js b/src/language-js/traverse/visitor-keys.evaluate.js
index f81071bde..820f1e693 100644
--- a/src/language-js/traverse/visitor-keys.evaluate.js
+++ b/src/language-js/traverse/visitor-keys.evaluate.js
@@ -38,6 +38,13 @@ const additionalVisitorKeys = {
   ],
   DeclareEnum: flowVisitorKeys.EnumDeclaration,
   InferTypeAnnotation: ["typeParameter"],
+  KeyofTypeAnnotation: ["argument"],
+  ObjectTypeMappedTypeProperty: [
+    "keyTparam",
+    "propType",
+    "sourceType",
+    "variance",
+  ],
   QualifiedTypeofIdentifier: ["id", "qualification"],
   TupleTypeAnnotation: ["elementTypes"],
   TupleTypeSpreadElement: ["label", "typeAnnotation"],
diff --git a/src/language-js/utils/index.js b/src/language-js/utils/index.js
index 8a59f76b9..4dc06fd12 100644
--- a/src/language-js/utils/index.js
+++ b/src/language-js/utils/index.js
@@ -485,19 +485,6 @@ function isSimpleTemplateLiteral(node) {
   });
 }
 
-/**
- * @param {string} tokenNode
- * @param {string} keyword
- * @returns {string}
- */
-function getTypeScriptMappedTypeModifier(tokenNode, keyword) {
-  if (tokenNode === "+" || tokenNode === "-") {
-    return tokenNode + keyword;
-  }
-
-  return keyword;
-}
-
 /**
  * @param {string} text
  * @param {Node} node
@@ -1131,7 +1118,6 @@ export {
   hasRestParameter,
   getLeftSide,
   getLeftSidePathName,
-  getTypeScriptMappedTypeModifier,
   hasLeadingOwnLineComment,
   hasNakedLeftSide,
   hasNode,
diff --git a/tests/format/flow/mapped-types/__snapshots__/jsfmt.spec.js.snap b/tests/format/flow/mapped-types/__snapshots__/jsfmt.spec.js.snap
new file mode 100644
index 000000000..aedba5a0d
--- /dev/null
+++ b/tests/format/flow/mapped-types/__snapshots__/jsfmt.spec.js.snap
@@ -0,0 +1,253 @@
+// Jest Snapshot v1, https://goo.gl/fbAQLP
+
+exports[`comments.js [babel-flow] format 1`] = `
+"Unexpected token, expected "]" (3:7)
+  1 | type Type = {
+  2 |   // comment
+> 3 |   +[T in number]: number;
+    |       ^
+  4 | };
+  5 |
+  6 | type Type = {"
+`;
+
+exports[`comments.js [babel-ts] format 1`] = `
+"Unexpected token (3:3)
+  1 | type Type = {
+  2 |   // comment
+> 3 |   +[T in number]: number;
+    |   ^
+  4 | };
+  5 |
+  6 | type Type = {"
+`;
+
+exports[`comments.js [typescript] format 1`] = `
+"Property or signature expected. (3:3)
+  1 | type Type = {
+  2 |   // comment
+> 3 |   +[T in number]: number;
+    |   ^
+  4 | };
+  5 |
+  6 | type Type = {"
+`;
+
+exports[`comments.js format 1`] = `
+====================================options=====================================
+parsers: ["flow", "typescript"]
+printWidth: 80
+                                                                                | printWidth
+=====================================input======================================
+type Type = {
+  // comment
+  +[T in number]: number;
+};
+
+type Type = {
+  // comment1
+  // comment2
+  +[T in number]: number;
+};
+
+type Type = {
+  // comment
+  -[T in number]: number;
+};
+
+type Type = {
+  // comment
+  +    [T in number]: number;
+};
+
+type Type = {
+  // comment
+  +     [T in number]: number;
+};
+
+type Type = {
+  // comment
+  +       [T in number]: number;
+};
+
+type Type = {
+  // comment
+  [T in number]: number;
+};
+
+type Type = {
+  + // foo
+  /* bar */ [T in number]: number;
+};
+
+=====================================output=====================================
+type Type = {
+  // comment
+  +[T in number]: number,
+};
+
+type Type = {
+  // comment1
+  // comment2
+  +[T in number]: number,
+};
+
+type Type = {
+  // comment
+  -[T in number]: number,
+};
+
+type Type = {
+  // comment
+  +[T in number]: number,
+};
+
+type Type = {
+  // comment
+  +[T in number]: number,
+};
+
+type Type = {
+  // comment
+  +[T in number]: number,
+};
+
+type Type = {
+  // comment
+  [T in number]: number,
+};
+
+type Type = {
+  +[/* bar */ T in number]: number, // foo
+};
+
+================================================================================
+`;
+
+exports[`mapped-types.js [babel-flow] format 1`] = `
+"Unexpected token, expected "]" (3:21)
+  1 | //@flow
+  2 |
+> 3 | type Mapped = {[key in keyof O]: O[key]};
+    |                     ^
+  4 | type MappedLong = {[key in keyof AReallyLongNameThatShouldReallyMostDefinitelyCauseALineWrap]: number};
+  5 | type MappedWithVariance = {+[key in keyof O]: number};
+  6 | type MappedWithVarianceLong = {-[key in keyof AReallyLongNameThatShouldReallyMostDefinitelyCauseALineWrap]: number};"
+`;
+
+exports[`mapped-types.js [babel-ts] format 1`] = `
+"Unexpected token (5:28)
+  3 | type Mapped = {[key in keyof O]: O[key]};
+  4 | type MappedLong = {[key in keyof AReallyLongNameThatShouldReallyMostDefinitelyCauseALineWrap]: number};
+> 5 | type MappedWithVariance = {+[key in keyof O]: number};
+    |                            ^
+  6 | type MappedWithVarianceLong = {-[key in keyof AReallyLongNameThatShouldReallyMostDefinitelyCauseALineWrap]: number};
+  7 | type MappedWithPlusOptional = {[key in keyof O]+?: number};
+  8 | type MappedWithMinusOptional = {[key in keyof O]-?: number};"
+`;
+
+exports[`mapped-types.js [typescript] format 1`] = `
+"Property or signature expected. (5:28)
+  3 | type Mapped = {[key in keyof O]: O[key]};
+  4 | type MappedLong = {[key in keyof AReallyLongNameThatShouldReallyMostDefinitelyCauseALineWrap]: number};
+> 5 | type MappedWithVariance = {+[key in keyof O]: number};
+    |                            ^
+  6 | type MappedWithVarianceLong = {-[key in keyof AReallyLongNameThatShouldReallyMostDefinitelyCauseALineWrap]: number};
+  7 | type MappedWithPlusOptional = {[key in keyof O]+?: number};
+  8 | type MappedWithMinusOptional = {[key in keyof O]-?: number};"
+`;
+
+exports[`mapped-types.js format 1`] = `
+====================================options=====================================
+parsers: ["flow", "typescript"]
+printWidth: 80
+                                                                                | printWidth
+=====================================input======================================
+//@flow
+
+type Mapped = {[key in keyof O]: O[key]};
+type MappedLong = {[key in keyof AReallyLongNameThatShouldReallyMostDefinitelyCauseALineWrap]: number};
+type MappedWithVariance = {+[key in keyof O]: number};
+type MappedWithVarianceLong = {-[key in keyof AReallyLongNameThatShouldReallyMostDefinitelyCauseALineWrap]: number};
+type MappedWithPlusOptional = {[key in keyof O]+?: number};
+type MappedWithMinusOptional = {[key in keyof O]-?: number};
+type MappedWithOptional = {[key in keyof O]?: number};
+type MappedWithPlusOptionalLong = {[key in keyof AReallyLongNameThatShouldReallyMostDefinitelyCauseALineWrap]+?: number};
+type MappedWithMinusOptionalLong = {[key in keyof AReallyLongNameThatShouldReallyMostDefinitelyCauseALineWrap]-?: number};
+type MappedWithOptionalLong = {[key in keyof AReallyLongNameThatShouldReallyMostDefinitelyCauseALineWrap]?: number};
+type MappedWithOptionalAndVariance = {+[key in keyof O]+?: number};
+type MappedWithOptionalAndVarianceLong = {+[key in keyof AReallyLongNameThatShouldReallyMostDefinitelyCauseALineWrap]+?: number};
+type MappedWithComments = {/*comment about variance */+[key /* comment about key name */ in /* comment before source type */ keyof O /* comment after source type */]/* comment about optionality */? /*comment before colon */:number /*comment about the prop type */};
+
+=====================================output=====================================
+//@flow
+
+type Mapped = { [key in keyof O]: O[key] };
+type MappedLong = {
+  [key in keyof AReallyLongNameThatShouldReallyMostDefinitelyCauseALineWrap]: number,
+};
+type MappedWithVariance = { +[key in keyof O]: number };
+type MappedWithVarianceLong = {
+  -[key in keyof AReallyLongNameThatShouldReallyMostDefinitelyCauseALineWrap]: number,
+};
+type MappedWithPlusOptional = { [key in keyof O]+?: number };
+type MappedWithMinusOptional = { [key in keyof O]-?: number };
+type MappedWithOptional = { [key in keyof O]?: number };
+type MappedWithPlusOptionalLong = {
+  [key in keyof AReallyLongNameThatShouldReallyMostDefinitelyCauseALineWrap]+?: number,
+};
+type MappedWithMinusOptionalLong = {
+  [key in keyof AReallyLongNameThatShouldReallyMostDefinitelyCauseALineWrap]-?: number,
+};
+type MappedWithOptionalLong = {
+  [key in keyof AReallyLongNameThatShouldReallyMostDefinitelyCauseALineWrap]?: number,
+};
+type MappedWithOptionalAndVariance = { +[key in keyof O]+?: number };
+type MappedWithOptionalAndVarianceLong = {
+  +[key in keyof AReallyLongNameThatShouldReallyMostDefinitelyCauseALineWrap]+?: number,
+};
+type MappedWithComments = {
+  /*comment about variance */ +[key /* comment about key name */ in /* comment before source type */ keyof O /* comment after source type */ /* comment about optionality */ /*comment before colon */]?: number /*comment about the prop type */,
+};
+
+================================================================================
+`;
+
+exports[`ts-compatibility.js [babel-flow] format 1`] = `
+"Unexpected token, expected "]" (3:19)
+  1 | // Tests that on single-line mapped types Flow and TS make the same formatting decisions
+  2 |
+> 3 | type Test = {[key in T]: number};
+    |                   ^
+  4 | type Test = {[key in keyof T]: number};
+  5 | type Test = {[otherKeyName     in keyof T]: number};
+  6 | type Test = {[key in T]:number};"
+`;
+
+exports[`ts-compatibility.js format 1`] = `
+====================================options=====================================
+parsers: ["flow", "typescript"]
+printWidth: 80
+                                                                                | printWidth
+=====================================input======================================
+// Tests that on single-line mapped types Flow and TS make the same formatting decisions
+
+type Test = {[key in T]: number};
+type Test = {[key in keyof T]: number};
+type Test = {[otherKeyName     in keyof T]: number};
+type Test = {[key in T]:number};
+type Test = {[key in T]+?:number};
+type Test = {[key in T]-?:                   number};
+
+=====================================output=====================================
+// Tests that on single-line mapped types Flow and TS make the same formatting decisions
+
+type Test = { [key in T]: number };
+type Test = { [key in keyof T]: number };
+type Test = { [otherKeyName in keyof T]: number };
+type Test = { [key in T]: number };
+type Test = { [key in T]+?: number };
+type Test = { [key in T]-?: number };
+
+================================================================================
+`;
diff --git a/tests/format/flow/mapped-types/comments.js b/tests/format/flow/mapped-types/comments.js
new file mode 100644
index 000000000..fd51657a4
--- /dev/null
+++ b/tests/format/flow/mapped-types/comments.js
@@ -0,0 +1,40 @@
+type Type = {
+  // comment
+  +[T in number]: number;
+};
+
+type Type = {
+  // comment1
+  // comment2
+  +[T in number]: number;
+};
+
+type Type = {
+  // comment
+  -[T in number]: number;
+};
+
+type Type = {
+  // comment
+  +    [T in number]: number;
+};
+
+type Type = {
+  // comment
+  +     [T in number]: number;
+};
+
+type Type = {
+  // comment
+  +       [T in number]: number;
+};
+
+type Type = {
+  // comment
+  [T in number]: number;
+};
+
+type Type = {
+  + // foo
+  /* bar */ [T in number]: number;
+};
diff --git a/tests/format/flow/mapped-types/jsfmt.spec.js b/tests/format/flow/mapped-types/jsfmt.spec.js
new file mode 100644
index 000000000..066cf3d1f
--- /dev/null
+++ b/tests/format/flow/mapped-types/jsfmt.spec.js
@@ -0,0 +1,7 @@
+run_spec(import.meta, ["flow", "typescript"], {
+  errors: {
+    "babel-flow": ["comments.js", "mapped-types.js", "ts-compatibility.js"],
+    "babel-ts": ["comments.js", "mapped-types.js"],
+    typescript: ["comments.js", "mapped-types.js"],
+  },
+});
diff --git a/tests/format/flow/mapped-types/mapped-types.js b/tests/format/flow/mapped-types/mapped-types.js
new file mode 100644
index 000000000..5be422501
--- /dev/null
+++ b/tests/format/flow/mapped-types/mapped-types.js
@@ -0,0 +1,15 @@
+//@flow
+
+type Mapped = {[key in keyof O]: O[key]};
+type MappedLong = {[key in keyof AReallyLongNameThatShouldReallyMostDefinitelyCauseALineWrap]: number};
+type MappedWithVariance = {+[key in keyof O]: number};
+type MappedWithVarianceLong = {-[key in keyof AReallyLongNameThatShouldReallyMostDefinitelyCauseALineWrap]: number};
+type MappedWithPlusOptional = {[key in keyof O]+?: number};
+type MappedWithMinusOptional = {[key in keyof O]-?: number};
+type MappedWithOptional = {[key in keyof O]?: number};
+type MappedWithPlusOptionalLong = {[key in keyof AReallyLongNameThatShouldReallyMostDefinitelyCauseALineWrap]+?: number};
+type MappedWithMinusOptionalLong = {[key in keyof AReallyLongNameThatShouldReallyMostDefinitelyCauseALineWrap]-?: number};
+type MappedWithOptionalLong = {[key in keyof AReallyLongNameThatShouldReallyMostDefinitelyCauseALineWrap]?: number};
+type MappedWithOptionalAndVariance = {+[key in keyof O]+?: number};
+type MappedWithOptionalAndVarianceLong = {+[key in keyof AReallyLongNameThatShouldReallyMostDefinitelyCauseALineWrap]+?: number};
+type MappedWithComments = {/*comment about variance */+[key /* comment about key name */ in /* comment before source type */ keyof O /* comment after source type */]/* comment about optionality */? /*comment before colon */:number /*comment about the prop type */};
diff --git a/tests/format/flow/mapped-types/ts-compatibility.js b/tests/format/flow/mapped-types/ts-compatibility.js
new file mode 100644
index 000000000..a9453b7c4
--- /dev/null
+++ b/tests/format/flow/mapped-types/ts-compatibility.js
@@ -0,0 +1,8 @@
+// Tests that on single-line mapped types Flow and TS make the same formatting decisions
+
+type Test = {[key in T]: number};
+type Test = {[key in keyof T]: number};
+type Test = {[otherKeyName     in keyof T]: number};
+type Test = {[key in T]:number};
+type Test = {[key in T]+?:number};
+type Test = {[key in T]-?:                   number};
diff --git a/tests/unit/__snapshots__/visitor-keys.js.snap b/tests/unit/__snapshots__/visitor-keys.js.snap
index fda608497..2652e35e0 100644
--- a/tests/unit/__snapshots__/visitor-keys.js.snap
+++ b/tests/unit/__snapshots__/visitor-keys.js.snap
@@ -446,6 +446,9 @@ exports[`visitor keys estree 1`] = `
   "JsonRoot": [
     "node",
   ],
+  "KeyofTypeAnnotation": [
+    "argument",
+  ],
   "LabeledStatement": [
     "label",
     "body",
@@ -563,6 +566,12 @@ exports[`visitor keys estree 1`] = `
     "static",
     "method",
   ],
+  "ObjectTypeMappedTypeProperty": [
+    "keyTparam",
+    "propType",
+    "sourceType",
+    "variance",
+  ],
   "ObjectTypeProperty": [
     "key",
     "value",