diff --git a/Libraries/Network/XMLHttpRequest.js b/Libraries/Network/XMLHttpRequest.js
index c12888f087a146..31581f95648f46 100644
--- a/Libraries/Network/XMLHttpRequest.js
+++ b/Libraries/Network/XMLHttpRequest.js
@@ -423,12 +423,44 @@ class XMLHttpRequest extends (EventTarget(...XHR_EVENTS): any) {
       // according to the spec, return null if no response has been received
       return null;
     }
-    const headers = this.responseHeaders || {};
-    return Object.keys(headers)
-      .map(headerName => {
-        return headerName + ': ' + headers[headerName];
-      })
-      .join('\r\n');
+
+    // Assign to non-nullable local variable.
+    const responseHeaders = this.responseHeaders;
+
+    const unsortedHeaders = {};
+    for (const rawHeaderName of Object.keys(responseHeaders)) {
+      const headerValue = responseHeaders[rawHeaderName];
+      const lowerHeaderName = rawHeaderName.toLowerCase();
+      if (unsortedHeaders.hasOwnProperty(lowerHeaderName)) {
+        unsortedHeaders[lowerHeaderName].headerValue += ', ' + headerValue;
+      } else {
+        unsortedHeaders[lowerHeaderName] = {
+          lowerHeaderName,
+          upperHeaderName: rawHeaderName.toUpperCase(),
+          headerValue,
+        };
+      }
+    }
+
+    // Sort in ascending order, with a being less than b if a's name is legacy-uppercased-byte less than b's name.
+    const sortedHeaders = Object.values(unsortedHeaders).sort((a, b) => {
+      if (a.upperHeaderName < b.upperHeaderName) {
+        return -1;
+      }
+      if (a.upperHeaderName > b.upperHeaderName) {
+        return 1;
+      }
+      return 0;
+    });
+
+    // Combine into single text response.
+    return (
+      sortedHeaders
+        .map(header => {
+          return header.lowerHeaderName + ': ' + header.headerValue;
+        })
+        .join('\r\n') + '\r\n'
+    );
   }
 
   getResponseHeader(header: string): ?string {
diff --git a/Libraries/Network/__tests__/XMLHttpRequest-test.js b/Libraries/Network/__tests__/XMLHttpRequest-test.js
index 91d2ce1e252c98..f769d356f78881 100644
--- a/Libraries/Network/__tests__/XMLHttpRequest-test.js
+++ b/Libraries/Network/__tests__/XMLHttpRequest-test.js
@@ -241,7 +241,7 @@ describe('XMLHttpRequest', function() {
     });
 
     expect(xhr.getAllResponseHeaders()).toBe(
-      'Content-Type: text/plain; charset=utf-8\r\n' + 'Content-Length: 32',
+      'content-length: 32\r\n' + 'content-type: text/plain; charset=utf-8\r\n',
     );
   });
 
@@ -292,4 +292,22 @@ describe('XMLHttpRequest', function() {
     );
     expect(GlobalPerformanceLogger.stopTimespan).not.toHaveBeenCalled();
   });
+
+  it('should sort and lowercase response headers', function() {
+    // Derived from XHR Web Platform Test: https://github.com/web-platform-tests/wpt/blob/master/xhr/getallresponseheaders.htm
+    xhr.open('GET', 'blabla');
+    xhr.send();
+    setRequestId(10);
+    xhr.__didReceiveResponse(requestId, 200, {
+      'foo-TEST': '1',
+      'FOO-test': '2',
+      __Custom: 'token',
+      'ALSO-here': 'Mr. PB',
+      ewok: 'lego',
+    });
+
+    expect(xhr.getAllResponseHeaders()).toBe(
+      'also-here: Mr. PB\r\newok: lego\r\nfoo-test: 1, 2\r\n__custom: token\r\n',
+    );
+  });
 });