diff --git a/Tests/LibURL/TestURL.cpp b/Tests/LibURL/TestURL.cpp
index 89cc014d273..7548aa8ff3d 100644
--- a/Tests/LibURL/TestURL.cpp
+++ b/Tests/LibURL/TestURL.cpp
@@ -489,3 +489,24 @@ TEST_CASE(username_and_password)
         EXPECT_EQ(MUST(url.password()), password);
     }
 }
+
+TEST_CASE(ascii_only_url)
+{
+    {
+        constexpr auto upper_case_url = "HTTP://EXAMPLE.COM:80/INDEX.HTML#FRAGMENT"sv;
+        URL::URL url(upper_case_url);
+        EXPECT(url.is_valid());
+        EXPECT_EQ(url.scheme(), "http");
+        EXPECT_EQ(MUST(url.serialized_host()), "example.com"sv);
+        EXPECT_EQ(url.to_byte_string(), "http://example.com/INDEX.HTML#FRAGMENT");
+    }
+
+    {
+        constexpr auto mixed_case_url = "hTtP://eXaMpLe.CoM:80/iNdEx.HtMl#fRaGmEnT"sv;
+        URL::URL url(mixed_case_url);
+        EXPECT(url.is_valid());
+        EXPECT_EQ(url.scheme(), "http");
+        EXPECT_EQ(MUST(url.serialized_host()), "example.com"sv);
+        EXPECT_EQ(url.to_byte_string(), "http://example.com/iNdEx.HtMl#fRaGmEnT");
+    }
+}
diff --git a/Userland/Libraries/LibURL/Parser.cpp b/Userland/Libraries/LibURL/Parser.cpp
index d91843dfb55..55b483a6571 100644
--- a/Userland/Libraries/LibURL/Parser.cpp
+++ b/Userland/Libraries/LibURL/Parser.cpp
@@ -586,7 +586,9 @@ static ErrorOr<String> domain_to_ascii(StringView domain, bool be_strict)
         // 3. If result is the empty string, domain-to-ASCII validation error, return failure.
         if (domain.is_empty())
             return Error::from_string_literal("Empty domain");
-        return String::from_utf8_without_validation(domain.bytes());
+
+        auto lowercase_domain = domain.to_lowercase_string();
+        return String::from_utf8_without_validation(lowercase_domain.bytes());
     }
 
     Unicode::IDNA::ToAsciiOptions const options {