diff --git a/Tests/LibWeb/Text/expected/Streams/ReadableStream-default-tee.txt b/Tests/LibWeb/Text/expected/Streams/ReadableStream-default-tee.txt
new file mode 100644
index 00000000000..daeb8e4bb1e
--- /dev/null
+++ b/Tests/LibWeb/Text/expected/Streams/ReadableStream-default-tee.txt
@@ -0,0 +1,8 @@
+stream1: abcdefghijklmnopqrstuvwxyz
+stream1: ABCDEFGHIJKLMNOPQRSTUVWXYZ
+stream1: 0123456789!@#$%^&*()-=_+,<
+stream1: Done!
+stream2: abcdefghijklmnopqrstuvwxyz
+stream2: ABCDEFGHIJKLMNOPQRSTUVWXYZ
+stream2: 0123456789!@#$%^&*()-=_+,<
+stream2: Done!
diff --git a/Tests/LibWeb/Text/input/Streams/ReadableStream-default-tee.html b/Tests/LibWeb/Text/input/Streams/ReadableStream-default-tee.html
new file mode 100644
index 00000000000..01eca132f26
--- /dev/null
+++ b/Tests/LibWeb/Text/input/Streams/ReadableStream-default-tee.html
@@ -0,0 +1,56 @@
+<script src="../include.js"></script>
+<script>
+    const CHUNK1 = "abcdefghijklmnopqrstuvwxyz";
+    const CHUNK2 = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";
+    const CHUNK3 = "0123456789!@#$%^&*()-=_+,<";
+
+    const readStream = (stream, name) => {
+        const reader = stream.getReader();
+
+        return new Promise((resolve, reject) => {
+            const processText = ({ done, value }) => {
+                if (done) {
+                    println(`${name}: Done!`);
+                    resolve();
+                    return;
+                }
+
+                println(`${name}: ${value}`);
+
+                return reader.read().then(processText);
+            };
+
+            reader.read().then(processText);
+        });
+    };
+
+    asyncTest(done => {
+        const stream = new ReadableStream({
+            start(controller) {
+                pullCount = 0;
+            },
+
+            pull(controller) {
+                ++pullCount;
+
+                if (pullCount == 1) {
+                    controller.enqueue(CHUNK1);
+                } else if (pullCount == 2) {
+                    controller.enqueue(CHUNK2);
+                } else if (pullCount == 3) {
+                    controller.enqueue(CHUNK3);
+                } else {
+                    controller.close();
+                }
+            },
+
+            cancel() {},
+        });
+
+        const teed = stream.tee();
+
+        readStream(teed[0], "stream1").then(() => {
+            readStream(teed[1], "stream2").then(done);
+        });
+    });
+</script>
diff --git a/Userland/Libraries/LibWeb/Streams/AbstractOperations.cpp b/Userland/Libraries/LibWeb/Streams/AbstractOperations.cpp
index 38d5e969548..fcc9f29f65e 100644
--- a/Userland/Libraries/LibWeb/Streams/AbstractOperations.cpp
+++ b/Userland/Libraries/LibWeb/Streams/AbstractOperations.cpp
@@ -7,6 +7,7 @@
  * SPDX-License-Identifier: BSD-2-Clause
  */
 
+#include <LibJS/Runtime/Array.h>
 #include <LibJS/Runtime/ArrayBuffer.h>
 #include <LibJS/Runtime/DataView.h>
 #include <LibJS/Runtime/DataViewConstructor.h>
@@ -16,6 +17,8 @@
 #include <LibJS/Runtime/TypedArray.h>
 #include <LibWeb/Bindings/ExceptionOrUtils.h>
 #include <LibWeb/DOM/AbortSignal.h>
+#include <LibWeb/HTML/EventLoop/EventLoop.h>
+#include <LibWeb/HTML/Scripting/TemporaryExecutionContext.h>
 #include <LibWeb/HTML/StructuredSerialize.h>
 #include <LibWeb/Streams/AbstractOperations.h>
 #include <LibWeb/Streams/QueuingStrategy.h>
@@ -239,7 +242,7 @@ bool readable_stream_has_default_reader(ReadableStream const& stream)
 }
 
 // https://streams.spec.whatwg.org/#readable-stream-tee
-WebIDL::ExceptionOr<ReadableStreamPair> readable_stream_tee(JS::Realm& realm, ReadableStream& stream, bool)
+WebIDL::ExceptionOr<ReadableStreamPair> readable_stream_tee(JS::Realm& realm, ReadableStream& stream, bool clone_for_branch2)
 {
     // 1. Assert: stream implements ReadableStream.
     // 2. Assert: cloneForBranch2 is a boolean.
@@ -250,7 +253,308 @@ WebIDL::ExceptionOr<ReadableStreamPair> readable_stream_tee(JS::Realm& realm, Re
     }
 
     // 4. Return ? ReadableStreamDefaultTee(stream, cloneForBranch2).
-    return realm.vm().throw_completion<JS::InternalError>(JS::ErrorType::NotImplemented, "Default stream teeing");
+    return TRY(readable_stream_default_tee(realm, stream, clone_for_branch2));
+}
+
+struct DefaultStreamTeeParams final : JS::Cell {
+    JS_CELL(TeeParams, JS::Cell);
+    JS_DECLARE_ALLOCATOR(DefaultStreamTeeParams);
+
+    virtual void visit_edges(Visitor& visitor) override
+    {
+        Base::visit_edges(visitor);
+        visitor.visit(reason1);
+        visitor.visit(reason2);
+        visitor.visit(branch1);
+        visitor.visit(branch2);
+        visitor.visit(pull_algorithm);
+    }
+
+    bool reading { false };
+    bool read_again { false };
+    bool canceled1 { false };
+    bool canceled2 { false };
+    JS::Value reason1 { JS::js_undefined() };
+    JS::Value reason2 { JS::js_undefined() };
+    JS::GCPtr<ReadableStream> branch1;
+    JS::GCPtr<ReadableStream> branch2;
+    JS::GCPtr<PullAlgorithm> pull_algorithm;
+};
+
+JS_DEFINE_ALLOCATOR(DefaultStreamTeeParams);
+
+// https://streams.spec.whatwg.org/#ref-for-read-request③
+class DefaultStreamTeeReadRequest final : public ReadRequest {
+    JS_CELL(DefaultStreamTeeReadRequest, Cell);
+    JS_DECLARE_ALLOCATOR(DefaultStreamTeeReadRequest);
+
+public:
+    DefaultStreamTeeReadRequest(
+        JS::Realm& realm,
+        JS::NonnullGCPtr<ReadableStream> stream,
+        JS::NonnullGCPtr<DefaultStreamTeeParams> params,
+        JS::NonnullGCPtr<WebIDL::Promise> cancel_promise,
+        bool clone_for_branch2)
+        : m_realm(realm)
+        , m_stream(stream)
+        , m_params(params)
+        , m_cancel_promise(cancel_promise)
+        , m_clone_for_branch2(clone_for_branch2)
+    {
+    }
+
+    // https://streams.spec.whatwg.org/#ref-for-read-request-chunk-steps③
+    virtual void on_chunk(JS::Value chunk) override
+    {
+        // 1. Queue a microtask to perform the following steps:
+        HTML::queue_a_microtask(nullptr, [this, chunk]() {
+            HTML::TemporaryExecutionContext execution_context { Bindings::host_defined_environment_settings_object(m_realm) };
+
+            auto controller1 = m_params->branch1->controller()->get<JS::NonnullGCPtr<ReadableStreamDefaultController>>();
+            auto controller2 = m_params->branch2->controller()->get<JS::NonnullGCPtr<ReadableStreamDefaultController>>();
+
+            // 1. Set readAgain to false.
+            m_params->read_again = false;
+
+            // 2. Let chunk1 and chunk2 be chunk.
+            auto chunk1 = chunk;
+            auto chunk2 = chunk;
+
+            // 3. If canceled2 is false and cloneForBranch2 is true,
+            if (!m_params->canceled2 && m_clone_for_branch2) {
+                // 1. Let cloneResult be StructuredClone(chunk2).
+                auto clone_result = structured_clone(m_realm, chunk2);
+
+                // 2. If cloneResult is an abrupt completion,
+                if (clone_result.is_exception()) {
+                    auto completion = Bindings::dom_exception_to_throw_completion(m_realm->vm(), clone_result.release_error());
+
+                    // 1. Perform ! ReadableStreamDefaultControllerError(branch1.[[controller]], cloneResult.[[Value]]).
+                    readable_stream_default_controller_error(controller1, completion.value().value());
+
+                    // 2. Perform ! ReadableStreamDefaultControllerError(branch2.[[controller]], cloneResult.[[Value]]).
+                    readable_stream_default_controller_error(controller2, completion.value().value());
+
+                    // 3. Resolve cancelPromise with ! ReadableStreamCancel(stream, cloneResult.[[Value]]).
+                    auto cancel_result = MUST(readable_stream_cancel(m_stream, completion.value().value()));
+                    JS::NonnullGCPtr cancel_value = verify_cast<JS::Promise>(*cancel_result->promise().ptr());
+
+                    WebIDL::resolve_promise(m_realm, m_cancel_promise, cancel_value);
+
+                    // 4. Return.
+                    return;
+                }
+
+                // 3. Otherwise, set chunk2 to cloneResult.[[Value]].
+                chunk2 = clone_result.release_value();
+            }
+
+            // 4. If canceled1 is false, perform ! ReadableStreamDefaultControllerEnqueue(branch1.[[controller]], chunk1).
+            if (!m_params->canceled1) {
+                MUST(readable_stream_default_controller_enqueue(controller1, chunk1));
+            }
+
+            // 5. If canceled2 is false, perform ! ReadableStreamDefaultControllerEnqueue(branch2.[[controller]], chunk2).
+            if (!m_params->canceled2) {
+                MUST(readable_stream_default_controller_enqueue(controller2, chunk2));
+            }
+
+            // 6. Set reading to false.
+            m_params->reading = false;
+
+            // 7. If readAgain is true, perform pullAlgorithm.
+            if (m_params->read_again) {
+                MUST(m_params->pull_algorithm->function()());
+            }
+        });
+
+        // NOTE: The microtask delay here is necessary because it takes at least a microtask to detect errors, when we
+        //       use reader.[[closedPromise]] below. We want errors in stream to error both branches immediately, so we
+        //       cannot let successful synchronously-available reads happen ahead of asynchronously-available errors.
+    }
+
+    // https://streams.spec.whatwg.org/#ref-for-read-request-close-steps②
+    virtual void on_close() override
+    {
+        auto controller1 = m_params->branch1->controller()->get<JS::NonnullGCPtr<ReadableStreamDefaultController>>();
+        auto controller2 = m_params->branch2->controller()->get<JS::NonnullGCPtr<ReadableStreamDefaultController>>();
+
+        // 1. Set reading to false.
+        m_params->reading = false;
+
+        // 2. If canceled1 is false, perform ! ReadableStreamDefaultControllerClose(branch1.[[controller]]).
+        if (!m_params->canceled1) {
+            readable_stream_default_controller_close(controller1);
+        }
+
+        // 3. If canceled2 is false, perform ! ReadableStreamDefaultControllerClose(branch2.[[controller]]).
+        if (!m_params->canceled2) {
+            readable_stream_default_controller_close(controller2);
+        }
+
+        // 4. If canceled1 is false or canceled2 is false, resolve cancelPromise with undefined.
+        if (!m_params->canceled1 || !m_params->canceled2) {
+            WebIDL::resolve_promise(m_realm, m_cancel_promise, JS::js_undefined());
+        }
+    }
+
+    // https://streams.spec.whatwg.org/#ref-for-read-request-error-steps③
+    virtual void on_error(JS::Value) override
+    {
+        // 1. Set reading to false.
+        m_params->reading = false;
+    }
+
+private:
+    virtual void visit_edges(Visitor& visitor) override
+    {
+        Base::visit_edges(visitor);
+        visitor.visit(m_realm);
+        visitor.visit(m_stream);
+        visitor.visit(m_params);
+        visitor.visit(m_cancel_promise);
+    }
+
+    JS::NonnullGCPtr<JS::Realm> m_realm;
+    JS::NonnullGCPtr<ReadableStream> m_stream;
+    JS::NonnullGCPtr<DefaultStreamTeeParams> m_params;
+    JS::NonnullGCPtr<WebIDL::Promise> m_cancel_promise;
+    bool m_clone_for_branch2 { false };
+};
+
+JS_DEFINE_ALLOCATOR(DefaultStreamTeeReadRequest);
+
+// https://streams.spec.whatwg.org/#abstract-opdef-readablestreamdefaulttee
+WebIDL::ExceptionOr<ReadableStreamPair> readable_stream_default_tee(JS::Realm& realm, ReadableStream& stream, bool clone_for_branch2)
+{
+    // 1. Assert: stream implements ReadableStream.
+    // 2. Assert: cloneForBranch2 is a boolean.
+
+    // 3. Let reader be ? AcquireReadableStreamDefaultReader(stream).
+    auto reader = TRY(acquire_readable_stream_default_reader(stream));
+
+    // 4. Let reading be false.
+    // 5. Let readAgain be false.
+    // 6. Let canceled1 be false.
+    // 7. Let canceled2 be false.
+    // 8. Let reason1 be undefined.
+    // 9. Let reason2 be undefined.
+    // 10. Let branch1 be undefined.
+    // 11. Let branch2 be undefined.
+    auto params = realm.heap().allocate<DefaultStreamTeeParams>(realm);
+
+    // 12. Let cancelPromise be a new promise.
+    auto cancel_promise = WebIDL::create_promise(realm);
+
+    // 13. Let pullAlgorithm be the following steps:
+    auto pull_algorithm = JS::create_heap_function(realm.heap(), [&realm, &stream, reader, params, cancel_promise, clone_for_branch2]() -> WebIDL::ExceptionOr<JS::NonnullGCPtr<WebIDL::Promise>> {
+        // 1. If reading is true,
+        if (params->reading) {
+            // 1. Set readAgain to true.
+            params->read_again = true;
+
+            // 2. Return a promise resolved with undefined.
+            return WebIDL::create_resolved_promise(realm, JS::js_undefined());
+        }
+
+        // 2. Set reading to true.
+        params->reading = true;
+
+        // 3. Let readRequest be a read request with the following items:
+        auto read_request = realm.heap().allocate_without_realm<DefaultStreamTeeReadRequest>(realm, stream, params, cancel_promise, clone_for_branch2);
+
+        // 4. Perform ! ReadableStreamDefaultReaderRead(reader, readRequest).
+        MUST(readable_stream_default_reader_read(reader, read_request));
+
+        // 5. Return a promise resolved with undefined.
+        return WebIDL::create_resolved_promise(realm, JS::js_undefined());
+    });
+
+    // AD-HOC: The read request within the pull algorithm must be able to re-invoke the pull algorithm, so cache it here.
+    params->pull_algorithm = pull_algorithm;
+
+    // 14. Let cancel1Algorithm be the following steps, taking a reason argument:
+    auto cancel1_algorithm = JS::create_heap_function(realm.heap(), [&realm, &stream, params, cancel_promise](JS::Value reason) -> WebIDL::ExceptionOr<JS::NonnullGCPtr<WebIDL::Promise>> {
+        // 1. Set canceled1 to true.
+        params->canceled1 = true;
+
+        // 2. Set reason1 to reason.
+        params->reason1 = reason;
+
+        // 3. If canceled2 is true,
+        if (params->canceled2) {
+            // 1. Let compositeReason be ! CreateArrayFromList(« reason1, reason2 »).
+            auto composite_reason = JS::Array::create_from(realm, AK::Array { params->reason1, params->reason2 });
+
+            // 2. Let cancelResult be ! ReadableStreamCancel(stream, compositeReason).
+            auto cancel_result = MUST(readable_stream_cancel(stream, composite_reason));
+
+            // 3. Resolve cancelPromise with cancelResult.
+            JS::NonnullGCPtr cancel_value = verify_cast<JS::Promise>(*cancel_result->promise().ptr());
+            WebIDL::resolve_promise(realm, cancel_promise, cancel_value);
+        }
+
+        // 4. Return cancelPromise.
+        return cancel_promise;
+    });
+
+    // 15. Let cancel2Algorithm be the following steps, taking a reason argument:
+    auto cancel2_algorithm = JS::create_heap_function(realm.heap(), [&realm, &stream, params, cancel_promise](JS::Value reason) -> WebIDL::ExceptionOr<JS::NonnullGCPtr<WebIDL::Promise>> {
+        // 1. Set canceled2 to true.
+        params->canceled2 = true;
+
+        // 2. Set reason2 to reason.
+        params->reason2 = reason;
+
+        // 3. If canceled1 is true,
+        if (params->canceled1) {
+            // 1. Let compositeReason be ! CreateArrayFromList(« reason1, reason2 »).
+            auto composite_reason = JS::Array::create_from(realm, AK::Array { params->reason1, params->reason2 });
+
+            // 2. Let cancelResult be ! ReadableStreamCancel(stream, compositeReason).
+            auto cancel_result = MUST(readable_stream_cancel(stream, composite_reason));
+
+            // 3. Resolve cancelPromise with cancelResult.
+            JS::NonnullGCPtr cancel_value = verify_cast<JS::Promise>(*cancel_result->promise().ptr());
+            WebIDL::resolve_promise(realm, cancel_promise, cancel_value);
+        }
+
+        // 4. Return cancelPromise.
+        return cancel_promise;
+    });
+
+    // 16. Let startAlgorithm be an algorithm that returns undefined.
+    auto start_algorithm = JS::create_heap_function(realm.heap(), []() -> WebIDL::ExceptionOr<JS::Value> {
+        return JS::js_undefined();
+    });
+
+    // 17. Set branch1 to ! CreateReadableStream(startAlgorithm, pullAlgorithm, cancel1Algorithm).
+    params->branch1 = MUST(create_readable_stream(realm, start_algorithm, pull_algorithm, cancel1_algorithm));
+
+    // 18. Set branch2 to ! CreateReadableStream(startAlgorithm, pullAlgorithm, cancel2Algorithm).
+    params->branch2 = MUST(create_readable_stream(realm, start_algorithm, pull_algorithm, cancel2_algorithm));
+
+    // 19. Upon rejection of reader.[[closedPromise]] with reason r,
+    WebIDL::upon_rejection(*reader->closed_promise_capability(), [&realm, params, cancel_promise](auto reason) -> WebIDL::ExceptionOr<JS::Value> {
+        auto controller1 = params->branch1->controller()->get<JS::NonnullGCPtr<ReadableStreamDefaultController>>();
+        auto controller2 = params->branch2->controller()->get<JS::NonnullGCPtr<ReadableStreamDefaultController>>();
+
+        // 1. Perform ! ReadableStreamDefaultControllerError(branch1.[[controller]], r).
+        readable_stream_default_controller_error(controller1, reason);
+
+        // 2. Perform ! ReadableStreamDefaultControllerError(branch2.[[controller]], r).
+        readable_stream_default_controller_error(controller2, reason);
+
+        // 3. If canceled1 is false or canceled2 is false, resolve cancelPromise with undefined.
+        if (!params->canceled1 || !params->canceled2) {
+            WebIDL::resolve_promise(realm, cancel_promise, JS::js_undefined());
+        }
+
+        return JS::js_undefined();
+    });
+
+    // 20. Return « branch1, branch2 ».
+    return ReadableStreamPair { *params->branch1, *params->branch2 };
 }
 
 // https://streams.spec.whatwg.org/#make-size-algorithm-from-size-function
diff --git a/Userland/Libraries/LibWeb/Streams/AbstractOperations.h b/Userland/Libraries/LibWeb/Streams/AbstractOperations.h
index f24c0b1bb13..ab63a65104b 100644
--- a/Userland/Libraries/LibWeb/Streams/AbstractOperations.h
+++ b/Userland/Libraries/LibWeb/Streams/AbstractOperations.h
@@ -48,6 +48,7 @@ bool readable_stream_has_byob_reader(ReadableStream const&);
 bool readable_stream_has_default_reader(ReadableStream const&);
 
 WebIDL::ExceptionOr<ReadableStreamPair> readable_stream_tee(JS::Realm&, ReadableStream&, bool clone_for_branch2);
+WebIDL::ExceptionOr<ReadableStreamPair> readable_stream_default_tee(JS::Realm& realm, ReadableStream& stream, bool clone_for_branch2);
 
 WebIDL::ExceptionOr<JS::NonnullGCPtr<WebIDL::Promise>> readable_stream_reader_generic_cancel(ReadableStreamGenericReaderMixin&, JS::Value reason);
 void readable_stream_reader_generic_initialize(ReadableStreamReader, ReadableStream&);