diff --git a/dom/base/URLSearchParams.cpp b/dom/base/URLSearchParams.cpp
index ceec58d1e89f26f1d23f1e9f72d738e2c41abc10..7e7c1df7b042ef3fb98bac42e6b68150aab3f1e9 100644
--- a/dom/base/URLSearchParams.cpp
+++ b/dom/base/URLSearchParams.cpp
@@ -301,13 +301,6 @@ URLSearchParams::Delete(const nsAString& aName)
   NotifyObserver();
 }
 
-uint32_t
-URLSearchParams::Size()
-{
-  Validate();
-  return mSearchParams.Count();
-}
-
 void
 URLSearchParams::DeleteAll()
 {
diff --git a/dom/base/URLSearchParams.h b/dom/base/URLSearchParams.h
index fa2b9090ee0dccc901183bd4731f5fd3ca3c96b9..0106a7dcedab5bc854eee4af570258e4cc418dae 100644
--- a/dom/base/URLSearchParams.h
+++ b/dom/base/URLSearchParams.h
@@ -85,8 +85,6 @@ public:
 
   void Delete(const nsAString& aName);
 
-  uint32_t Size();
-
   void Stringify(nsString& aRetval)
   {
     Validate();
diff --git a/dom/base/test/test_urlSearchParams.html b/dom/base/test/test_urlSearchParams.html
index 89dc0bf738ede7075ed4445bb6b0b49c1651314c..d8c02ecaabc3ab62a1bd5f386311112b2cf6b5d3 100644
--- a/dom/base/test/test_urlSearchParams.html
+++ b/dom/base/test/test_urlSearchParams.html
@@ -32,23 +32,19 @@ https://bugzilla.mozilla.org/show_bug.cgi?id=887836
     is(u.has('foo'), false, 'URLSearchParams.has(foo)');
     is(u.get('foo'), '', 'URLSearchParams.get(foo)');
     is(u.getAll('foo').length, 0, 'URLSearchParams.getAll(foo)');
-    is(u.size, 0, 'URLSearchParams.size()');
 
     u.append('foo', 'bar');
     is(u.has('foo'), true, 'URLSearchParams.has(foo)');
     is(u.get('foo'), 'bar', 'URLSearchParams.get(foo)');
     is(u.getAll('foo').length, 1, 'URLSearchParams.getAll(foo)');
-    is(u.size, 1, 'URLSearchParams.size()');
 
     u.set('foo', 'bar2');
     is(u.get('foo'), 'bar2', 'URLSearchParams.get(foo)');
     is(u.getAll('foo').length, 1, 'URLSearchParams.getAll(foo)');
-    is(u.size, 1, 'URLSearchParams.size()');
 
     is(u + "", "foo=bar2", "stringifier");
 
     u.delete('foo');
-    is(u.size, 0, 'URLSearchParams.size()');
 
     runTest();
   }
@@ -57,10 +53,8 @@ https://bugzilla.mozilla.org/show_bug.cgi?id=887836
     var u = new URLSearchParams();
     ok(u, "URLSearchParams created");
     u.append('foo', 'bar');
-    is(u.size, 1, "u.size()");
 
     var uu = new URLSearchParams(u);
-    is(uu.size, 1, "uu.size()");
     is(uu.get('foo'), 'bar', 'uu.get()');
 
     u.append('foo', 'bar2');
@@ -114,8 +108,6 @@ https://bugzilla.mozilla.org/show_bug.cgi?id=887836
           is(all[k], checks[i].data[key][k], "value matches");
         }
       }
-
-      is(u.size, count, "size matches");
     }
 
     runTest();
diff --git a/dom/webidl/URLSearchParams.webidl b/dom/webidl/URLSearchParams.webidl
index 9a693e3d4e25434828f6b72f10f5034320f28a6c..74ef3aa889abb669546841a268bc9257a4a390bd 100644
--- a/dom/webidl/URLSearchParams.webidl
+++ b/dom/webidl/URLSearchParams.webidl
@@ -22,6 +22,5 @@ interface URLSearchParams {
   sequence<DOMString> getAll(DOMString name);
   boolean has(DOMString name);
   void set(DOMString name, DOMString value);
-  readonly attribute unsigned long size;
   stringifier;
 };
diff --git a/dom/workers/test/urlSearchParams_worker.js b/dom/workers/test/urlSearchParams_worker.js
index f2b4bbb6e699b666c5532febbb6b95153f1eb4f2..ce972a5adeb187d902e14ff8853ee39dc9586af7 100644
--- a/dom/workers/test/urlSearchParams_worker.js
+++ b/dom/workers/test/urlSearchParams_worker.js
@@ -29,23 +29,19 @@ onmessage = function() {
     is(u.has('foo'), false, 'URLSearchParams.has(foo)');
     is(u.get('foo'), '', 'URLSearchParams.get(foo)');
     is(u.getAll('foo').length, 0, 'URLSearchParams.getAll(foo)');
-    is(u.size, 0, 'URLSearchParams.size()');
 
     u.append('foo', 'bar');
     is(u.has('foo'), true, 'URLSearchParams.has(foo)');
     is(u.get('foo'), 'bar', 'URLSearchParams.get(foo)');
     is(u.getAll('foo').length, 1, 'URLSearchParams.getAll(foo)');
-    is(u.size, 1, 'URLSearchParams.size()');
 
     u.set('foo', 'bar2');
     is(u.get('foo'), 'bar2', 'URLSearchParams.get(foo)');
     is(u.getAll('foo').length, 1, 'URLSearchParams.getAll(foo)');
-    is(u.size, 1, 'URLSearchParams.size()');
 
     is(u + "", "foo=bar2", "stringify");
 
     u.delete('foo');
-    is(u.size, 0, 'URLSearchParams.size()');
 
     runTest();
   }
@@ -54,10 +50,8 @@ onmessage = function() {
     var u = new URLSearchParams();
     ok(u, "URLSearchParams created");
     u.append('foo', 'bar');
-    is(u.size, 1, "u.size()");
 
     var uu = new URLSearchParams(u);
-    is(uu.size, 1, "uu.size()");
     is(uu.get('foo'), 'bar', 'uu.get()');
 
     u.append('foo', 'bar2');
@@ -111,8 +105,6 @@ onmessage = function() {
           is(all[k], checks[i].data[key][k], "value matches");
         }
       }
-
-      is(u.size, count, "size matches");
     }
 
     runTest();