diff --git a/Libraries/LibURL/CMakeLists.txt b/Libraries/LibURL/CMakeLists.txt
index 8b41bdc67c9..304b623a05f 100644
--- a/Libraries/LibURL/CMakeLists.txt
+++ b/Libraries/LibURL/CMakeLists.txt
@@ -4,6 +4,7 @@ set(SOURCES
     Host.cpp
     Origin.cpp
     Parser.cpp
+    Site.cpp
     URL.cpp
     ${PUBLIC_SUFFIX_SOURCES}
 )
diff --git a/Libraries/LibURL/Forward.h b/Libraries/LibURL/Forward.h
index ec9da1963cb..58c0a8ad7e7 100644
--- a/Libraries/LibURL/Forward.h
+++ b/Libraries/LibURL/Forward.h
@@ -7,9 +7,11 @@
 #pragma once
 
 namespace URL {
+class Host;
 class Origin;
-class URL;
 class Parser;
+class Site;
+class URL;
 
 struct BlobURLEntry;
 }
diff --git a/Libraries/LibURL/Origin.cpp b/Libraries/LibURL/Origin.cpp
index 0042d7a7515..9d14bac6675 100644
--- a/Libraries/LibURL/Origin.cpp
+++ b/Libraries/LibURL/Origin.cpp
@@ -6,9 +6,27 @@
 
 #include <LibURL/Origin.h>
 #include <LibURL/Parser.h>
+#include <LibURL/Site.h>
 
 namespace URL {
 
+// https://html.spec.whatwg.org/multipage/browsers.html#same-site
+bool Origin::is_same_site(Origin const& other) const
+{
+    // 1. Let siteA be the result of obtaining a site given A.
+    auto site_a = Site::obtain(*this);
+
+    // 2. Let siteB be the result of obtaining a site given B.
+    auto site_b = Site::obtain(other);
+
+    // 3. If siteA is same site with siteB, then return true.
+    if (site_a.is_same_site(site_b))
+        return true;
+
+    // 4. Return false.
+    return false;
+}
+
 // https://html.spec.whatwg.org/multipage/origin.html#ascii-serialisation-of-an-origin
 String Origin::serialize() const
 {
diff --git a/Libraries/LibURL/Origin.h b/Libraries/LibURL/Origin.h
index 64ebe68604b..ecc0cb81c9f 100644
--- a/Libraries/LibURL/Origin.h
+++ b/Libraries/LibURL/Origin.h
@@ -8,6 +8,7 @@
 #pragma once
 
 #include <AK/ByteString.h>
+#include <LibURL/Forward.h>
 #include <LibURL/Host.h>
 
 namespace URL {
@@ -74,6 +75,9 @@ public:
         return false;
     }
 
+    // https://html.spec.whatwg.org/multipage/browsers.html#same-site
+    bool is_same_site(Origin const&) const;
+
     // https://html.spec.whatwg.org/multipage/origin.html#ascii-serialisation-of-an-origin
     String serialize() const;
 
diff --git a/Libraries/LibURL/Site.cpp b/Libraries/LibURL/Site.cpp
new file mode 100644
index 00000000000..9f097f6f547
--- /dev/null
+++ b/Libraries/LibURL/Site.cpp
@@ -0,0 +1,83 @@
+/*
+ * Copyright (c) 2024, Sam Atkins <sam@ladybird.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <LibURL/Site.h>
+
+namespace URL {
+
+// https://html.spec.whatwg.org/multipage/browsers.html#obtain-a-site
+Site Site::obtain(Origin const& origin)
+{
+    // 1. If origin is an opaque origin, then return origin.
+    if (origin.is_opaque())
+        return Site { origin };
+
+    // 2. If origin's host's registrable domain is null, then return (origin's scheme, origin's host).
+    auto origin_scheme = origin.scheme().value_or_lazy_evaluated([]() { return String {}; });
+
+    auto registrable_domain = origin.host().registrable_domain();
+    if (!registrable_domain.has_value())
+        return Site { SchemeAndHost { origin_scheme, origin.host() } };
+
+    // 3. Return (origin's scheme, origin's host's registrable domain).
+    return Site { SchemeAndHost { origin_scheme, Host { *registrable_domain } } };
+}
+
+Site::Site(Variant<Origin, SchemeAndHost> value)
+    : m_value(move(value))
+{
+}
+
+// https://html.spec.whatwg.org/multipage/browsers.html#concept-site-same-site
+bool Site::is_same_site(Site const& other) const
+{
+    // 1. If A and B are the same opaque origin, then return true.
+    // NOTE: Origins in sites are always opaque.
+    // FIXME: Currently all opaque origins are identical, how should we distinguish them?
+    if (m_value.has<Origin>() && other.m_value.has<Origin>())
+        return true;
+
+    // 2. If A or B is an opaque origin, then return false.
+    if (m_value.has<Origin>() || other.m_value.has<Origin>())
+        return false;
+
+    // 3. If A's and B's scheme values are different, then return false.
+    auto& a = m_value.get<SchemeAndHost>();
+    auto& b = other.m_value.get<SchemeAndHost>();
+    if (a.scheme != b.scheme)
+        return false;
+
+    // 4. If A's and B's host values are not equal, then return false.
+    if (a.host != b.host)
+        return false;
+
+    // 5. Return true.
+    return true;
+}
+
+// https://html.spec.whatwg.org/multipage/browsers.html#serialization-of-a-site
+String Site::serialize() const
+{
+    // 1. If site is an opaque origin, then return "null".
+    if (m_value.has<Origin>())
+        return "null"_string;
+
+    // 2. Let result be site[0].
+    auto& [scheme, host] = m_value.get<SchemeAndHost>();
+    StringBuilder result;
+    result.append(scheme);
+
+    // 3. Append "://" to result.
+    result.append("://"sv);
+
+    // 4. Append site[1], serialized, to result.
+    result.append(host.serialize());
+
+    // 5. Return result.
+    return result.to_string_without_validation();
+}
+
+}
diff --git a/Libraries/LibURL/Site.h b/Libraries/LibURL/Site.h
new file mode 100644
index 00000000000..2010a0bca6f
--- /dev/null
+++ b/Libraries/LibURL/Site.h
@@ -0,0 +1,37 @@
+/*
+ * Copyright (c) 2024, Sam Atkins <sam@ladybird.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/Variant.h>
+#include <LibURL/Host.h>
+#include <LibURL/Origin.h>
+
+namespace URL {
+
+// https://html.spec.whatwg.org/multipage/browsers.html#scheme-and-host
+// A scheme-and-host is a tuple of a scheme (an ASCII string) and a host (a host).
+struct SchemeAndHost {
+    String scheme;
+    Host host;
+};
+
+// https://html.spec.whatwg.org/multipage/browsers.html#site
+// A site is an opaque origin or a scheme-and-host.
+class Site {
+public:
+    static Site obtain(Origin const&);
+
+    bool is_same_site(Site const& other) const;
+
+    String serialize() const;
+
+private:
+    Site(Variant<Origin, SchemeAndHost>);
+    Variant<Origin, SchemeAndHost> m_value;
+};
+
+}