diff --git a/lib/internal/url.js b/lib/internal/url.js
index 8c04ac70c07c..f089ff3439c9 100644
--- a/lib/internal/url.js
+++ b/lib/internal/url.js
@@ -23,6 +23,7 @@ const {
   StringPrototypeIndexOf,
   StringPrototypeSlice,
   StringPrototypeStartsWith,
+  StringPrototypeToUpperCase,
   StringPrototypeToWellFormed,
   Symbol,
   SymbolIterator,
@@ -1685,7 +1686,8 @@ function pathToFileURL(filepath, options = kEmptyObject) {
     // Handle extended UNC path and standard UNC path
     // "\\?\UNC\" path prefix should be ignored.
     // Ref: https://learn.microsoft.com/en-us/windows/win32/fileio/maximum-file-path-limitation
-    const isExtendedUNC = StringPrototypeStartsWith(resolved, '\\\\?\\UNC\\');
+    const isExtendedUNC = StringPrototypeStartsWith(resolved, '\\\\?\\') &&
+      StringPrototypeToUpperCase(StringPrototypeSlice(resolved, 4, 8)) === 'UNC\\';
     const prefixLength = isExtendedUNC ? 8 : 2;
     const hostnameEndIndex = StringPrototypeIndexOf(resolved, '\\', prefixLength);
     if (hostnameEndIndex === -1) {
@@ -1695,7 +1697,7 @@ function pathToFileURL(filepath, options = kEmptyObject) {
         'Missing UNC resource path',
       );
     }
-    if (hostnameEndIndex === 2) {
+    if (hostnameEndIndex === prefixLength) {
       throw new ERR_INVALID_ARG_VALUE(
         'path',
         resolved,
diff --git a/test/parallel/test-url-pathtofileurl.js b/test/parallel/test-url-pathtofileurl.js
index 290cac41037c..816bf7377eca 100644
--- a/test/parallel/test-url-pathtofileurl.js
+++ b/test/parallel/test-url-pathtofileurl.js
@@ -95,6 +95,22 @@ const url = require('url');
       code: 'ERR_INVALID_ARG_VALUE',
     });
 
+    // Missing server in extended UNC:
+    assert.throws(() => url.pathToFileURL('\\\\?\\UNC\\\\no-server'), {
+      code: 'ERR_INVALID_ARG_VALUE',
+    });
+    assert.throws(() => url.pathToFileURL('\\\\?\\unc\\\\no-server'), {
+      code: 'ERR_INVALID_ARG_VALUE',
+    });
+
+    // Missing share or resource in extended UNC:
+    assert.throws(() => url.pathToFileURL('\\\\?\\UNC\\host'), {
+      code: 'ERR_INVALID_ARG_VALUE',
+    });
+    assert.throws(() => url.pathToFileURL('\\\\?\\unc\\host'), {
+      code: 'ERR_INVALID_ARG_VALUE',
+    });
+
     // Regression test for direct String.prototype.startsWith call
     assert.throws(() => url.pathToFileURL([
       '\\\\',
@@ -173,6 +189,10 @@ const windowsTestCases = [
   { path: '\\\\nas\\My Docs\\File.doc', expected: 'file://nas/My%20Docs/File.doc' },
   // Extended UNC path
   { path: '\\\\?\\UNC\\server\\share\\folder\\file.txt', expected: 'file://server/share/folder/file.txt' },
+  // Lowercase extended UNC path
+  { path: '\\\\?\\unc\\server\\share\\folder\\file.txt', expected: 'file://server/share/folder/file.txt' },
+  // Mixed-case extended UNC path
+  { path: '\\\\?\\Unc\\server\\share\\folder\\file.txt', expected: 'file://server/share/folder/file.txt' },
 ];
 const alphabet = String.fromCharCode(...Array.from({ length: 26 }, (_, i) => 'a'.charCodeAt() + i));
 const posixTestCases = [