diff --git a/Libraries/LibURL/CMakeLists.txt b/Libraries/LibURL/CMakeLists.txt
index 63f9bd7d469..9c72b4165cf 100644
--- a/Libraries/LibURL/CMakeLists.txt
+++ b/Libraries/LibURL/CMakeLists.txt
@@ -10,6 +10,7 @@ set(SOURCES
     Pattern/Canonicalization.cpp
     Pattern/ConstructorStringParser.cpp
     Pattern/Pattern.cpp
+    Pattern/String.cpp
     Pattern/Tokenizer.cpp
 )
 
diff --git a/Libraries/LibURL/Pattern/String.cpp b/Libraries/LibURL/Pattern/String.cpp
new file mode 100644
index 00000000000..a829b12e9c6
--- /dev/null
+++ b/Libraries/LibURL/Pattern/String.cpp
@@ -0,0 +1,49 @@
+/*
+ * Copyright (c) 2025, Shannon Booth <shannon@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <AK/StringBuilder.h>
+#include <LibURL/Pattern/String.h>
+
+namespace URL::Pattern {
+
+// https://urlpattern.spec.whatwg.org/#escape-a-pattern-string
+String escape_a_pattern_string(String const& input)
+{
+    // 1. Assert: input is an ASCII string.
+    VERIFY(all_of(input.code_points(), is_ascii));
+
+    // 2. Let result be the empty string.
+    StringBuilder result;
+
+    // 3. Let index be 0.
+    // 4. While index is less than input’s length:
+    for (auto c : input.bytes_as_string_view()) {
+        // 1. Let c be input[index].
+        // 2. Increment index by 1.
+
+        // 3. If c is one of:
+        //     * U+002B (+);
+        //     * U+002A (*);
+        //     * U+003F (?);
+        //     * U+003A (:);
+        //     * U+007B ({);
+        //     * U+007D (});
+        //     * U+0028 (();
+        //     * U+0029 ()); or
+        //     * U+005C (\),
+        //    then append U+005C (\) to the end of result.
+        if ("+*?:{}()\\"sv.contains(c))
+            result.append('\\');
+
+        // 4. Append c to the end of result.
+        result.append(c);
+    }
+
+    // 5. Return result.
+    return result.to_string_without_validation();
+}
+
+}
diff --git a/Libraries/LibURL/Pattern/String.h b/Libraries/LibURL/Pattern/String.h
new file mode 100644
index 00000000000..dc617484f28
--- /dev/null
+++ b/Libraries/LibURL/Pattern/String.h
@@ -0,0 +1,15 @@
+/*
+ * Copyright (c) 2025, Shannon Booth <shannon@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/String.h>
+
+namespace URL::Pattern {
+
+String escape_a_pattern_string(String const&);
+
+}