diff --git a/changelog_unreleased/flow/19102.md b/changelog_unreleased/flow/19102.md
new file mode 100644
index 000000000..510ffb64d
--- /dev/null
+++ b/changelog_unreleased/flow/19102.md
@@ -0,0 +1,19 @@
+#### Add support for `writeonly`, `in`, and `out` variance modifiers (#19102 by @marcoww6)
+
+Support new Flow variance syntax: `writeonly` on object type properties/indexers, and `in T`/`out T` on type parameters.
+
+<!-- prettier-ignore -->
+```jsx
+// Input
+type T = {writeonly foo: string};
+type Contravariant<in T> = T;
+type Covariant<out T> = T;
+
+// Prettier stable
+// SyntaxError
+
+// Prettier main
+type T = { writeonly foo: string };
+type Contravariant<in T> = T;
+type Covariant<out T> = T;
+```
diff --git a/cspell.json b/cspell.json
index 55224441e..7d7dc3a8e 100644
--- a/cspell.json
+++ b/cspell.json
@@ -305,6 +305,7 @@
         "webstorm",
         "Weixin",
         "withastro",
+        "writeonly",
         "wxss",
         "xdescribe",
         "xiechao",
diff --git a/src/language-js/print/flow.js b/src/language-js/print/flow.js
index ad8e71d2d..9447bfd6e 100644
--- a/src/language-js/print/flow.js
+++ b/src/language-js/print/flow.js
@@ -218,7 +218,14 @@ function printFlow(path, options, print, args) {
       return ["?", print("typeAnnotation")];
     case "Variance": {
       const { kind } = node;
-      assert.ok(kind === "plus" || kind === "minus" || kind === "readonly");
+      assert.ok(
+        kind === "plus" ||
+          kind === "minus" ||
+          kind === "readonly" ||
+          kind === "writeonly" ||
+          kind === "in" ||
+          kind === "out",
+      );
       return kind === "plus" ? "+" : kind === "minus" ? "-" : `${kind} `;
     }
     case "KeyofTypeAnnotation":
diff --git a/tests/format/flow/in-out-variance/__snapshots__/format.test.js.snap b/tests/format/flow/in-out-variance/__snapshots__/format.test.js.snap
new file mode 100644
index 000000000..9b723f176
--- /dev/null
+++ b/tests/format/flow/in-out-variance/__snapshots__/format.test.js.snap
@@ -0,0 +1,38 @@
+// Jest Snapshot v1, https://jestjs.io/docs/snapshot-testing
+
+exports[`in-out-variance.js format 1`] = `
+====================================options=====================================
+parsers: ["flow"]
+                                                      printWidth: 80 (default) |
+=====================================input======================================
+type Contravariant<  in   T  > = T;
+
+type Covariant<  out   T  > = T;
+
+type Mixed<  in   T,   out   U  > = T;
+
+type InNamedIn<  in   in  > = void;
+
+type OutNamedOut<  out   out  > = void;
+
+type InWithBound<  in   T:   number  > = T;
+
+type OutWithDefault<  out   T  =   string  > = T;
+
+=====================================output=====================================
+type Contravariant<in T> = T;
+
+type Covariant<out T> = T;
+
+type Mixed<in T, out U> = T;
+
+type InNamedIn<in in> = void;
+
+type OutNamedOut<out out> = void;
+
+type InWithBound<in T: number> = T;
+
+type OutWithDefault<out T = string> = T;
+
+================================================================================
+`;
diff --git a/tests/format/flow/in-out-variance/format.test.js b/tests/format/flow/in-out-variance/format.test.js
new file mode 100644
index 000000000..8fbbcfa61
--- /dev/null
+++ b/tests/format/flow/in-out-variance/format.test.js
@@ -0,0 +1,5 @@
+runFormatTest(import.meta, ["flow"], {
+  errors: {
+    "babel-flow": true,
+  },
+});
diff --git a/tests/format/flow/in-out-variance/in-out-variance.js b/tests/format/flow/in-out-variance/in-out-variance.js
new file mode 100644
index 000000000..c20be233e
--- /dev/null
+++ b/tests/format/flow/in-out-variance/in-out-variance.js
@@ -0,0 +1,13 @@
+type Contravariant<  in   T  > = T;
+
+type Covariant<  out   T  > = T;
+
+type Mixed<  in   T,   out   U  > = T;
+
+type InNamedIn<  in   in  > = void;
+
+type OutNamedOut<  out   out  > = void;
+
+type InWithBound<  in   T:   number  > = T;
+
+type OutWithDefault<  out   T  =   string  > = T;
diff --git a/tests/format/flow/variance/__snapshots__/format.test.js.snap b/tests/format/flow/variance/__snapshots__/format.test.js.snap
index 6638d85f9..b92123817 100644
--- a/tests/format/flow/variance/__snapshots__/format.test.js.snap
+++ b/tests/format/flow/variance/__snapshots__/format.test.js.snap
@@ -24,6 +24,13 @@ type ReadonlyInterface = interface {
       readonly   prop:   string
 };
 
+type ReadonlyReservedWords = {
+  readonly   with:   string,
+  readonly   default?:   string,
+  readonly   enum:   number,
+  readonly   new:   boolean,
+};
+
 =====================================output=====================================
 class Route {
   static +param: T;
@@ -44,5 +51,12 @@ type ReadonlyInterface = interface {
   readonly prop: string,
 };
 
+type ReadonlyReservedWords = {
+  readonly with: string,
+  readonly default?: string,
+  readonly enum: number,
+  readonly new: boolean,
+};
+
 ================================================================================
 `;
diff --git a/tests/format/flow/variance/variance.js b/tests/format/flow/variance/variance.js
index 1858cd7d2..94c26eda0 100644
--- a/tests/format/flow/variance/variance.js
+++ b/tests/format/flow/variance/variance.js
@@ -16,3 +16,10 @@ type ReadonlyTuple = [  readonly   label:   number  ];
 type ReadonlyInterface = interface {
       readonly   prop:   string
 };
+
+type ReadonlyReservedWords = {
+  readonly   with:   string,
+  readonly   default?:   string,
+  readonly   enum:   number,
+  readonly   new:   boolean,
+};
diff --git a/tests/format/flow/writeonly/__snapshots__/format.test.js.snap b/tests/format/flow/writeonly/__snapshots__/format.test.js.snap
new file mode 100644
index 000000000..ebe67cd61
--- /dev/null
+++ b/tests/format/flow/writeonly/__snapshots__/format.test.js.snap
@@ -0,0 +1,30 @@
+// Jest Snapshot v1, https://jestjs.io/docs/snapshot-testing
+
+exports[`writeonly.js format 1`] = `
+====================================options=====================================
+parsers: ["flow"]
+                                                      printWidth: 80 (default) |
+=====================================input======================================
+type WriteonlyObj = {
+    writeonly   foo:   string,
+  writeonly [string]:   unknown
+};
+
+type WriteonlyReservedWords = {
+  writeonly   with:   string,
+  writeonly   default?:   string,
+};
+
+=====================================output=====================================
+type WriteonlyObj = {
+  writeonly foo: string,
+  writeonly [string]: unknown,
+};
+
+type WriteonlyReservedWords = {
+  writeonly with: string,
+  writeonly default?: string,
+};
+
+================================================================================
+`;
diff --git a/tests/format/flow/writeonly/format.test.js b/tests/format/flow/writeonly/format.test.js
new file mode 100644
index 000000000..8fbbcfa61
--- /dev/null
+++ b/tests/format/flow/writeonly/format.test.js
@@ -0,0 +1,5 @@
+runFormatTest(import.meta, ["flow"], {
+  errors: {
+    "babel-flow": true,
+  },
+});
diff --git a/tests/format/flow/writeonly/writeonly.js b/tests/format/flow/writeonly/writeonly.js
new file mode 100644
index 000000000..d8226ce67
--- /dev/null
+++ b/tests/format/flow/writeonly/writeonly.js
@@ -0,0 +1,9 @@
+type WriteonlyObj = {
+    writeonly   foo:   string,
+  writeonly [string]:   unknown
+};
+
+type WriteonlyReservedWords = {
+  writeonly   with:   string,
+  writeonly   default?:   string,
+};