diff --git a/Userland/Libraries/LibWeb/XHR/XMLHttpRequest.cpp b/Userland/Libraries/LibWeb/XHR/XMLHttpRequest.cpp
index 2ce8098312..9bd319bd1c 100644
--- a/Userland/Libraries/LibWeb/XHR/XMLHttpRequest.cpp
+++ b/Userland/Libraries/LibWeb/XHR/XMLHttpRequest.cpp
@@ -17,6 +17,7 @@
 #include <LibJS/Runtime/GlobalObject.h>
 #include <LibTextCodec/Decoder.h>
 #include <LibWeb/Bindings/EventWrapper.h>
+#include <LibWeb/Bindings/IDLAbstractOperations.h>
 #include <LibWeb/Bindings/XMLHttpRequestWrapper.h>
 #include <LibWeb/DOM/DOMException.h>
 #include <LibWeb/DOM/Document.h>
@@ -283,6 +284,11 @@ static ErrorOr<Fetch::Infrastructure::BodyWithType> extract_body(XMLHttpRequestB
                 type = blob->type().to_byte_buffer();
             return {};
         },
+        [&](JS::Handle<JS::Object> const& buffer_source) -> ErrorOr<void> {
+            // Set source to a copy of the bytes held by object.
+            source = TRY(Bindings::IDL::get_buffer_source_copy(*buffer_source.cell()));
+            return {};
+        },
         [&](NonnullRefPtr<URL::URLSearchParams> const& url_search_params) -> ErrorOr<void> {
             // Set source to the result of running the application/x-www-form-urlencoded serializer with object’s list.
             source = url_search_params->to_string().to_byte_buffer();
diff --git a/Userland/Libraries/LibWeb/XHR/XMLHttpRequest.h b/Userland/Libraries/LibWeb/XHR/XMLHttpRequest.h
index 2470323855..4380441417 100644
--- a/Userland/Libraries/LibWeb/XHR/XMLHttpRequest.h
+++ b/Userland/Libraries/LibWeb/XHR/XMLHttpRequest.h
@@ -24,7 +24,7 @@
 namespace Web::XHR {
 
 // https://fetch.spec.whatwg.org/#typedefdef-xmlhttprequestbodyinit
-using XMLHttpRequestBodyInit = Variant<NonnullRefPtr<FileAPI::Blob>, NonnullRefPtr<URL::URLSearchParams>, String>;
+using XMLHttpRequestBodyInit = Variant<NonnullRefPtr<FileAPI::Blob>, JS::Handle<JS::Object>, NonnullRefPtr<URL::URLSearchParams>, String>;
 
 class XMLHttpRequest final
     : public RefCounted<XMLHttpRequest>
diff --git a/Userland/Libraries/LibWeb/XHR/XMLHttpRequest.idl b/Userland/Libraries/LibWeb/XHR/XMLHttpRequest.idl
index 874006ea25..fa6b29a0c8 100644
--- a/Userland/Libraries/LibWeb/XHR/XMLHttpRequest.idl
+++ b/Userland/Libraries/LibWeb/XHR/XMLHttpRequest.idl
@@ -3,7 +3,7 @@
 #import <FileAPI/Blob.idl>
 #import <URL/URLSearchParams.idl>
 
-typedef (Blob or URLSearchParams or USVString) XMLHttpRequestBodyInit;
+typedef (Blob or BufferSource or URLSearchParams or USVString) XMLHttpRequestBodyInit;
 
 enum XMLHttpRequestResponseType {
   "",