diff --git a/ext/web/url.rs b/ext/web/url.rs
index 51ff5e3a0c975f..fca585f1c3032c 100644
--- a/ext/web/url.rs
+++ b/ext/web/url.rs
@@ -50,6 +50,37 @@ pub fn op_url_parse(
   parse_url(state, href, None, buf)
 }
 
+/// Detect Windows file paths like `C:\path\file.txt` and convert them to
+/// `file:///C:/path/file.txt` per the WHATWG URL spec change (url#874).
+///
+/// A Windows drive letter pattern is: a single ASCII alpha followed by `:`
+/// followed by `\`. This is detected in the "scheme start state" of the URL
+/// parser: the single letter would be parsed as a scheme, but the `:\`
+/// combination signals a Windows path instead.
+///
+/// Forward-slash drive paths (e.g. `C:/path`) are intentionally NOT converted:
+/// the `letter:/` shape is ambiguous with `letter:` schemes (e.g. `c:/foo`,
+/// `a://example.net`), and converting it would regress legitimate scheme URLs.
+/// Callers wanting forward-slash drive paths should pass `file:///C:/path`
+/// explicitly. This matches Node's current behavior.
+#[inline]
+fn maybe_convert_windows_path_to_file_url(href: &str) -> Option<String> {
+  let bytes = href.as_bytes();
+  // Must be at least 3 chars: letter, colon, backslash
+  if bytes.len() >= 3
+    && bytes[0].is_ascii_alphabetic()
+    && bytes[1] == b':'
+    && bytes[2] == b'\\'
+  {
+    let drive_letter = bytes[0] as char;
+    let rest = &href[3..];
+    let path = rest.replace('\\', "/");
+    Some(format!("file:///{drive_letter}:/{path}"))
+  } else {
+    None
+  }
+}
+
 /// `op_url_parse` and `op_url_parse_with_base` share the same implementation.
 ///
 /// This function is used to parse the URL and fill the `buf` with internal
@@ -81,7 +112,20 @@ fn parse_url(
   base_href: Option<&Url>,
   buf: &mut [u32],
 ) -> u32 {
-  match Url::options().base_url(base_href).parse(href) {
+  // Per WHATWG URL spec (url#874): detect Windows drive letter paths
+  // (e.g., "C:\path\file") and convert to file:/// URLs. The spec's
+  // scheme start state triggers this conversion regardless of base URL
+  // presence, since the single-letter "scheme" + `:` + `\` pattern
+  // unambiguously indicates a Windows drive path.
+  let converted;
+  let effective_href =
+    if let Some(file_url) = maybe_convert_windows_path_to_file_url(href) {
+      converted = file_url;
+      converted.as_str()
+    } else {
+      href
+    };
+  match Url::options().base_url(base_href).parse(effective_href) {
     Ok(url) => {
       let inner_url = quirks::internal_components(&url);
 
diff --git a/tests/unit/url_test.ts b/tests/unit/url_test.ts
index 2224d00665697a..292d35be8df5e7 100644
--- a/tests/unit/url_test.ts
+++ b/tests/unit/url_test.ts
@@ -527,3 +527,66 @@ Deno.test(function urlTakeURLObjectAsParameter() {
     "https://foo:bar@baz.qat:8000/qux/quux?foo=bar&baz=12#qat",
   );
 });
+
+// WHATWG URL spec change: Windows file path handling (url#874)
+Deno.test(function urlWindowsFilePathBasic() {
+  const url = new URL("C:\\path\\file.txt");
+  assertEquals(url.href, "file:///C:/path/file.txt");
+  assertEquals(url.protocol, "file:");
+  assertEquals(url.pathname, "/C:/path/file.txt");
+  assertEquals(url.host, "");
+});
+
+Deno.test(function urlWindowsFilePathDifferentDrives() {
+  assertEquals(new URL("D:\\foo\\bar.exe").href, "file:///D:/foo/bar.exe");
+  assertEquals(
+    new URL("Z:\\deep\\nested\\path.rs").href,
+    "file:///Z:/deep/nested/path.rs",
+  );
+});
+
+Deno.test(function urlWindowsFilePathLowercaseDrive() {
+  assertEquals(
+    new URL("c:\\folder\\file.txt").href,
+    "file:///c:/folder/file.txt",
+  );
+});
+
+Deno.test(function urlWindowsFilePathMixedSeparators() {
+  assertEquals(
+    new URL("C:\\path/mixed\\separators/file.txt").href,
+    "file:///C:/path/mixed/separators/file.txt",
+  );
+});
+
+Deno.test(function urlWindowsFilePathWithBase() {
+  const url = new URL("C:\\path\\file.node", "http://example.org/");
+  assertEquals(url.href, "file:///C:/path/file.node");
+  assertEquals(url.protocol, "file:");
+});
+
+Deno.test(function urlWindowsFilePathUrlParse() {
+  const url = URL.parse("C:\\folder\\file.txt");
+  assert(url !== null);
+  assertEquals(url.href, "file:///C:/folder/file.txt");
+  assertEquals(url.protocol, "file:");
+  assertEquals(url.pathname, "/C:/folder/file.txt");
+});
+
+Deno.test(function urlWindowsFilePathPercentEncodesSpaces() {
+  assertEquals(
+    new URL("C:\\path with space\\file.txt").href,
+    "file:///C:/path%20with%20space/file.txt",
+  );
+});
+
+// Regression guard: `letter:/...` and `letter://...` shapes must NOT be
+// rewritten to `file:///` — they are scheme URLs, not Windows drive paths.
+Deno.test(function urlSchemeAuthorityNotMisinterpretedAsDrive() {
+  assertEquals(new URL("a://example.net").href, "a://example.net");
+  assertEquals(new URL("h://.").href, "h://.");
+  assertEquals(new URL("w://x:0").href, "w://x:0");
+  // `c:/foo` is a `c:` scheme URL, not a Windows drive path. Forward-slash
+  // drive paths require explicit `file:///C:/foo`.
+  assertEquals(new URL("c:/foo").href, "c:/foo");
+});