diff --git a/changelog_unreleased/css/17841.md b/changelog_unreleased/css/17841.md
index 59d1efea8..c438b0932 100644
--- a/changelog_unreleased/css/17841.md
+++ b/changelog_unreleased/css/17841.md
@@ -1,19 +1,25 @@
-#### Handle attribute selector with case-sensitive flag (#17841 by @kovsu)
+#### Handle attribute selector with case-sensitive and uppercase case-insensitive flags (#17841, #17865 by @kovsu)
 
 <!-- prettier-ignore -->
 ```css
 /* Input */
-ol[type=a s] {
+[type=a s],
+[type=a S],
+[type=a I] {
   list-style-type: lower-alpha;
 }
 
 /* Prettier stable */
-ol[type="a s"] {
+[type="a s"],
+[type="a S"],
+[type="a I"] {
   list-style-type: lower-alpha;
 }
 
 /* Prettier main */
-ol[type="a" s] {
+[type="a" s],
+[type="a" S],
+[type="a" I] {
   list-style-type: lower-alpha;
 }
 ```
diff --git a/src/language-css/clean.js b/src/language-css/clean.js
index ac70c1da6..4508ebf13 100644
--- a/src/language-css/clean.js
+++ b/src/language-css/clean.js
@@ -121,8 +121,8 @@ function clean(original, cloned, parent) {
 
     if (original.value) {
       let { value } = cloned;
-      // Parser doesn't understand `s` or `S` flag
-      if (/\s[sS]$/u.test(value)) {
+      // Parser only understands the `i` flag
+      if (/\s[a-zA-Z]$/u.test(value)) {
         // Add an extra property to make sure flag is preserved
         cloned.__prettier_attribute_selector_flag = value.at(-1);
         value = value.slice(0, -1);
diff --git a/src/language-css/print/misc.js b/src/language-css/print/misc.js
index a2f4d6efd..e6ddc1a1d 100644
--- a/src/language-css/print/misc.js
+++ b/src/language-css/print/misc.js
@@ -28,9 +28,12 @@ function adjustStrings(value, options) {
 function quoteAttributeValue(value, options) {
   const quote = options.singleQuote ? "'" : '"';
 
-  // Check if the value ends with `s` or `S`, `i` can be solved by postcss
+  // The selector parser currently only understand `i` flag,
+  // but not `s`, `S`, and `I`
+  // To support future flags, we simply check if it's an alphabet letter
+  // https://github.com/prettier/prettier/pull/17865#discussion_r2332698101
   let flag = "";
-  const match = value.match(/^(?<value>.+?)\s+(?<flag>s|S)$/u);
+  const match = value.match(/^(?<value>.+?)\s+(?<flag>[a-zA-Z])$/u);
   if (match) {
     ({ value, flag } = match.groups);
   }
diff --git a/tests/format/css/attribute/__snapshots__/format.test.js.snap b/tests/format/css/attribute/__snapshots__/format.test.js.snap
index df5ead931..1c3e891e6 100644
--- a/tests/format/css/attribute/__snapshots__/format.test.js.snap
+++ b/tests/format/css/attribute/__snapshots__/format.test.js.snap
@@ -74,6 +74,13 @@ input[type="radio" i] {}
 img[alt~="person" i][src*="lorem" i] {}
 section:has(:not([type="radio" i], [type="checkbox" i])) {}
 
+input[type="radio" I] {}
+img[alt~="person" I][src*="lorem" I] {}
+section:has(:not([type="radio" I], [type="checkbox" I])) {}
+
+[type=a   I] {}
+[type="a"   I] {}
+
 =====================================output=====================================
 input[type='radio' i] {
 }
@@ -82,6 +89,18 @@ img[alt~='person' i][src*='lorem' i] {
 section:has(:not([type='radio' i], [type='checkbox' i])) {
 }
 
+input[type='radio' I] {
+}
+img[alt~='person' I][src*='lorem' I] {
+}
+section:has(:not([type='radio' I], [type='checkbox' I])) {
+}
+
+[type='a' I] {
+}
+[type='a' I] {
+}
+
 ================================================================================
 `;
 
@@ -95,6 +114,13 @@ input[type="radio" i] {}
 img[alt~="person" i][src*="lorem" i] {}
 section:has(:not([type="radio" i], [type="checkbox" i])) {}
 
+input[type="radio" I] {}
+img[alt~="person" I][src*="lorem" I] {}
+section:has(:not([type="radio" I], [type="checkbox" I])) {}
+
+[type=a   I] {}
+[type="a"   I] {}
+
 =====================================output=====================================
 input[type="radio" i] {
 }
@@ -103,6 +129,18 @@ img[alt~="person" i][src*="lorem" i] {
 section:has(:not([type="radio" i], [type="checkbox" i])) {
 }
 
+input[type="radio" I] {
+}
+img[alt~="person" I][src*="lorem" I] {
+}
+section:has(:not([type="radio" I], [type="checkbox" I])) {
+}
+
+[type="a" I] {
+}
+[type="a" I] {
+}
+
 ================================================================================
 `;
 
@@ -266,9 +304,9 @@ singleQuote: true
 [type='a' S] {
 }
 
-[type='a   x'] {
+[type='a' x] {
 }
-[type='a'   x] {
+[type='a' x] {
 }
 
 ================================================================================
@@ -307,9 +345,9 @@ printWidth: 80
 [type="a" S] {
 }
 
-[type="a   x"] {
+[type="a" x] {
 }
-[type="a"   x] {
+[type="a" x] {
 }
 
 ================================================================================
diff --git a/tests/format/css/attribute/insensitive.css b/tests/format/css/attribute/insensitive.css
index 46623f35d..ff157952d 100644
--- a/tests/format/css/attribute/insensitive.css
+++ b/tests/format/css/attribute/insensitive.css
@@ -1,3 +1,10 @@
 input[type="radio" i] {}
 img[alt~="person" i][src*="lorem" i] {}
 section:has(:not([type="radio" i], [type="checkbox" i])) {}
+
+input[type="radio" I] {}
+img[alt~="person" I][src*="lorem" I] {}
+section:has(:not([type="radio" I], [type="checkbox" I])) {}
+
+[type=a   I] {}
+[type="a"   I] {}