diff --git a/js/src/builtin/RegExp.cpp b/js/src/builtin/RegExp.cpp
index 2991118eb..909f58a0a 100644
--- a/js/src/builtin/RegExp.cpp
+++ b/js/src/builtin/RegExp.cpp
@@ -176,8 +176,11 @@ RegExpInitializeIgnoringLastIndex(JSContext* cx, Handle<RegExpObject*> obj,
     /* Steps 8-10. */
     CompileOptions options(cx);
     frontend::TokenStream dummyTokenStream(cx, options, nullptr, 0, nullptr);
-    if (!irregexp::ParsePatternSyntax(dummyTokenStream, cx->tempLifoAlloc(), pattern))
+    if (!irregexp::ParsePatternSyntax(dummyTokenStream, cx->tempLifoAlloc(), pattern,
+                                      flags & UnicodeFlag))
+    {   
         return false;
+    }
 
     if (staticsUse == UseRegExpStatics) {
         RegExpStatics* res = cx->global()->getRegExpStatics(cx);
@@ -561,6 +564,24 @@ regexp_sticky(JSContext* cx, unsigned argc, JS::Value* vp)
     return CallNonGenericMethod<IsRegExpObject, regexp_sticky_impl>(cx, args);
 }
 
+/* ES6 21.2.5.15. */
+MOZ_ALWAYS_INLINE bool
+regexp_unicode_impl(JSContext* cx, const CallArgs& args)
+{
+    MOZ_ASSERT(IsRegExpObject(args.thisv()));
+    /* Steps 4-6. */
+    args.rval().setBoolean(args.thisv().toObject().as<RegExpObject>().unicode());
+    return true;
+}
+
+static bool
+regexp_unicode(JSContext* cx, unsigned argc, JS::Value* vp)
+{
+    /* Steps 1-3. */
+    CallArgs args = CallArgsFromVp(argc, vp);
+    return CallNonGenericMethod<IsRegExpObject, regexp_unicode_impl>(cx, args);
+}
+
 const JSPropertySpec js::regexp_properties[] = {
     JS_SELF_HOSTED_GET("flags", "RegExpFlagsGetter", 0),
     JS_PSG("global", regexp_global, 0),
@@ -568,6 +589,7 @@ const JSPropertySpec js::regexp_properties[] = {
     JS_PSG("multiline", regexp_multiline, 0),
     JS_PSG("source", regexp_source, 0),
     JS_PSG("sticky", regexp_sticky, 0),
+    JS_PSG("unicode", regexp_unicode, 0),
     JS_PS_END
 };
 
diff --git a/js/src/builtin/RegExp.js b/js/src/builtin/RegExp.js
index 13cc5ba89..7b002dd47 100644
--- a/js/src/builtin/RegExp.js
+++ b/js/src/builtin/RegExp.js
@@ -25,9 +25,8 @@ function RegExpFlagsGetter() {
         result += "m";
 
     // Steps 13-15.
-    // TODO: Uncomment these steps when bug 1135377 is fixed.
-    // if (R.unicode)
-    //     result += "u";
+    if (R.unicode)
+         result += "u";
 
     // Steps 16-18.
     if (R.sticky)
diff --git a/js/src/frontend/TokenStream.cpp b/js/src/frontend/TokenStream.cpp
index 1570f4730..147c8128d 100644
--- a/js/src/frontend/TokenStream.cpp
+++ b/js/src/frontend/TokenStream.cpp
@@ -1582,6 +1582,8 @@ TokenStream::getTokenInternal(TokenKind* ttp, Modifier modifier)
                     reflags = RegExpFlag(reflags | MultilineFlag);
                 else if (c == 'y' && !(reflags & StickyFlag))
                     reflags = RegExpFlag(reflags | StickyFlag);
+                else if (c == 'u' && !(reflags & UnicodeFlag))
+                    reflags = RegExpFlag(reflags | UnicodeFlag);
                 else
                     break;
                 getChar();
diff --git a/js/src/irregexp/RegExpParser.cpp b/js/src/irregexp/RegExpParser.cpp
index e63092349..6ab53c59f 100644
--- a/js/src/irregexp/RegExpParser.cpp
+++ b/js/src/irregexp/RegExpParser.cpp
@@ -205,7 +205,8 @@ RegExpBuilder::AddQuantifierToAtom(int min, int max,
 
 template <typename CharT>
 RegExpParser<CharT>::RegExpParser(frontend::TokenStream& ts, LifoAlloc* alloc,
-                                  const CharT* chars, const CharT* end, bool multiline_mode)
+                                  const CharT* chars, const CharT* end, bool multiline_mode,
+                                  bool unicode)
   : ts(ts),
     alloc(alloc),
     captures_(nullptr),
@@ -215,6 +216,7 @@ RegExpParser<CharT>::RegExpParser(frontend::TokenStream& ts, LifoAlloc* alloc,
     capture_count_(0),
     has_more_(true),
     multiline_(multiline_mode),
+    unicode_(unicode),
     simple_(false),
     contains_anchor_(false),
     is_scanned_for_captures_(false)
@@ -1002,7 +1004,7 @@ template class irregexp::RegExpParser<char16_t>;
 template <typename CharT>
 static bool
 ParsePattern(frontend::TokenStream& ts, LifoAlloc& alloc, const CharT* chars, size_t length,
-             bool multiline, bool match_only, RegExpCompileData* data)
+             bool multiline, bool match_only, bool unicode, RegExpCompileData* data)
 {
     if (match_only) {
         // Try to strip a leading '.*' from the RegExp, but only if it is not
@@ -1025,7 +1027,7 @@ ParsePattern(frontend::TokenStream& ts, LifoAlloc& alloc, const CharT* chars, si
         }
     }
 
-    RegExpParser<CharT> parser(ts, &alloc, chars, chars + length, multiline);
+    RegExpParser<CharT> parser(ts, &alloc, chars, chars + length, multiline, unicode);
     data->tree = parser.ParsePattern();
     if (!data->tree)
         return false;
@@ -1038,32 +1040,34 @@ ParsePattern(frontend::TokenStream& ts, LifoAlloc& alloc, const CharT* chars, si
 
 bool
 irregexp::ParsePattern(frontend::TokenStream& ts, LifoAlloc& alloc, JSAtom* str,
-                       bool multiline, bool match_only,
+                       bool multiline, bool match_only, bool unicode,
                        RegExpCompileData* data)
 {
     JS::AutoCheckCannotGC nogc;
     return str->hasLatin1Chars()
            ? ::ParsePattern(ts, alloc, str->latin1Chars(nogc), str->length(),
-                            multiline, match_only, data)
+                            multiline, match_only, unicode, data)
            : ::ParsePattern(ts, alloc, str->twoByteChars(nogc), str->length(),
-                            multiline, match_only, data);
+                            multiline, match_only, unicode, data);
 }
 
 template <typename CharT>
 static bool
-ParsePatternSyntax(frontend::TokenStream& ts, LifoAlloc& alloc, const CharT* chars, size_t length)
+ParsePatternSyntax(frontend::TokenStream& ts, LifoAlloc& alloc, const CharT* chars, size_t length,
+                   bool unicode)
 {
     LifoAllocScope scope(&alloc);
 
-    RegExpParser<CharT> parser(ts, &alloc, chars, chars + length, false);
+    RegExpParser<CharT> parser(ts, &alloc, chars, chars + length, false, unicode);
     return parser.ParsePattern() != nullptr;
 }
 
 bool
-irregexp::ParsePatternSyntax(frontend::TokenStream& ts, LifoAlloc& alloc, JSAtom* str)
+irregexp::ParsePatternSyntax(frontend::TokenStream& ts, LifoAlloc& alloc, JSAtom* str,
+                             bool unicode)
 {
     JS::AutoCheckCannotGC nogc;
     return str->hasLatin1Chars()
-           ? ::ParsePatternSyntax(ts, alloc, str->latin1Chars(nogc), str->length())
-           : ::ParsePatternSyntax(ts, alloc, str->twoByteChars(nogc), str->length());
+           ? ::ParsePatternSyntax(ts, alloc, str->latin1Chars(nogc), str->length(), unicode)
+           : ::ParsePatternSyntax(ts, alloc, str->twoByteChars(nogc), str->length(), unicode);
 }
diff --git a/js/src/irregexp/RegExpParser.h b/js/src/irregexp/RegExpParser.h
index e36a47f75..740bff927 100644
--- a/js/src/irregexp/RegExpParser.h
+++ b/js/src/irregexp/RegExpParser.h
@@ -43,11 +43,12 @@ namespace irregexp {
 
 bool
 ParsePattern(frontend::TokenStream& ts, LifoAlloc& alloc, JSAtom* str,
-             bool multiline, bool match_only,
+             bool multiline, bool match_only, bool unicode,
              RegExpCompileData* data);
 
 bool
-ParsePatternSyntax(frontend::TokenStream& ts, LifoAlloc& alloc, JSAtom* str);
+ParsePatternSyntax(frontend::TokenStream& ts, LifoAlloc& alloc, JSAtom* str,
+                   bool unicode);
 
 // A BufferedVector is an automatically growing list, just like (and backed
 // by) a Vector, that is optimized for the case of adding and removing
@@ -174,7 +175,7 @@ class RegExpParser
 {
   public:
     RegExpParser(frontend::TokenStream& ts, LifoAlloc* alloc,
-                 const CharT* chars, const CharT* end, bool multiline_mode);
+                 const CharT* chars, const CharT* end, bool multiline_mode, bool unicode);
 
     RegExpTree* ParsePattern();
     RegExpTree* ParseDisjunction();
@@ -288,6 +289,7 @@ class RegExpParser
     int capture_count_;
     bool has_more_;
     bool multiline_;
+    bool unicode_;
     bool simple_;
     bool contains_anchor_;
     bool is_scanned_for_captures_;
diff --git a/js/src/jsapi.h b/js/src/jsapi.h
index 77aa95251..c094a7af0 100644
--- a/js/src/jsapi.h
+++ b/js/src/jsapi.h
@@ -4959,6 +4959,7 @@ JS_ObjectIsDate(JSContext* cx, JS::HandleObject obj, bool* isDate);
 #define JSREG_GLOB      0x02u   /* global exec, creates array of matches */
 #define JSREG_MULTILINE 0x04u   /* treat ^ and $ as begin and end of line */
 #define JSREG_STICKY    0x08u   /* only match starting at lastIndex */
+#define JSREG_UNICODE   0x10u   /* unicode */
 
 extern JS_PUBLIC_API(JSObject*)
 JS_NewRegExpObject(JSContext* cx, JS::HandleObject obj, const char* bytes, size_t length,
diff --git a/js/src/tests/ecma_6/RegExp/descriptor.js b/js/src/tests/ecma_6/RegExp/descriptor.js
index 62dda8521..cc545b3a6 100644
--- a/js/src/tests/ecma_6/RegExp/descriptor.js
+++ b/js/src/tests/ecma_6/RegExp/descriptor.js
@@ -10,7 +10,7 @@ var getters = [
   "multiline",
   "source",
   "sticky",
-  //"unicode",
+  "unicode",
 ];
 
 for (var name of getters) {
@@ -21,9 +21,5 @@ for (var name of getters) {
   assertEq("get" in desc, true);
 }
 
-// When the /u flag is supported, remove this comment and the next line, and
-// uncomment "unicode" in |props| above.
-assertThrowsInstanceOf(() => RegExp("", "mygui").flags, SyntaxError);
-
 if (typeof reportCompare === "function")
     reportCompare(true, true);
diff --git a/js/src/tests/ecma_6/RegExp/flag-accessors.js b/js/src/tests/ecma_6/RegExp/flag-accessors.js
index 4f169c8a4..848b916c5 100644
--- a/js/src/tests/ecma_6/RegExp/flag-accessors.js
+++ b/js/src/tests/ecma_6/RegExp/flag-accessors.js
@@ -8,17 +8,14 @@ var props = [
   "ignoreCase",
   "multiline",
   "sticky",
-  //"unicode",
+  "unicode",
 ];
 
 testThrows(RegExp.prototype);
 test(/foo/iymg, [true, true, true, true, false]);
 test(RegExp(""), [false, false, false, false, false]);
 test(RegExp("", "mygi"), [true, true, true, true, false]);
-// When the /u flag is supported, remove the following line, uncomment the
-// next line, and uncomment "unicode" in |props| above.
-assertThrowsInstanceOf(() => RegExp("", "mygui").flags, SyntaxError);
-// test(RegExp("", "mygiu"), [true, true, true, true, true]);
+test(RegExp("", "mygiu"), [true, true, true, true, true]);
 
 testThrowsGeneric();
 testThrowsGeneric(1);
diff --git a/js/src/tests/ecma_6/RegExp/flags.js b/js/src/tests/ecma_6/RegExp/flags.js
index 5a0245d99..292fa4f3f 100644
--- a/js/src/tests/ecma_6/RegExp/flags.js
+++ b/js/src/tests/ecma_6/RegExp/flags.js
@@ -7,16 +7,12 @@ assertEq(RegExp.prototype.flags, "");
 assertEq(/foo/iymg.flags, "gimy");
 assertEq(RegExp("").flags, "");
 assertEq(RegExp("", "mygi").flags, "gimy");
-// TODO: Uncomment lines 12, 16, 19 and remove lines 11, 15, 18 when bug 1135377 is fixed.
-assertThrowsInstanceOf(() => RegExp("", "mygui").flags, SyntaxError);
-// assertEq(RegExp("", "mygui").flags, "gimuy");
+assertEq(RegExp("", "mygui").flags, "gimuy");
 assertEq(genericFlags({}), "");
 assertEq(genericFlags({ignoreCase: true}), "i");
-assertEq(genericFlags({sticky:1, unicode:1, global: 0}), "y");
-// assertEq(genericFlags({sticky:1, unicode:1, global: 0}), "uy");
+assertEq(genericFlags({sticky:1, unicode:1, global: 0}), "uy");
 assertEq(genericFlags({__proto__: {multiline: true}}), "m");
-assertEq(genericFlags(new Proxy({}, {get(){return true}})), "gimy");
-// assertEq(genericFlags(new Proxy({}, {get(){return true}})), "gimuy");
+assertEq(genericFlags(new Proxy({}, {get(){return true}})), "gimuy");
 
 assertThrowsInstanceOf(() => genericFlags(), TypeError);
 assertThrowsInstanceOf(() => genericFlags(1), TypeError);
diff --git a/js/src/vm/RegExpObject.cpp b/js/src/vm/RegExpObject.cpp
index 9d0dd97d0..1faf252e7 100644
--- a/js/src/vm/RegExpObject.cpp
+++ b/js/src/vm/RegExpObject.cpp
@@ -39,6 +39,7 @@ JS_STATIC_ASSERT(IgnoreCaseFlag == JSREG_FOLD);
 JS_STATIC_ASSERT(GlobalFlag == JSREG_GLOB);
 JS_STATIC_ASSERT(MultilineFlag == JSREG_MULTILINE);
 JS_STATIC_ASSERT(StickyFlag == JSREG_STICKY);
+JS_STATIC_ASSERT(UnicodeFlag == JSREG_UNICODE);
 
 RegExpObject*
 js::RegExpAlloc(ExclusiveContext* cx, HandleObject proto /* = nullptr */)
@@ -219,7 +220,7 @@ RegExpObject::createNoStatics(ExclusiveContext* cx, HandleAtom source, RegExpFla
         tokenStream = dummyTokenStream.ptr();
     }
 
-    if (!irregexp::ParsePatternSyntax(*tokenStream, alloc, source))
+    if (!irregexp::ParsePatternSyntax(*tokenStream, alloc, source, flags & UnicodeFlag))
         return nullptr;
 
     Rooted<RegExpObject*> regexp(cx, RegExpAlloc(cx));
@@ -267,6 +268,7 @@ RegExpObject::initIgnoringLastIndex(HandleAtom source, RegExpFlag flags)
     setIgnoreCase(flags & IgnoreCaseFlag);
     setMultiline(flags & MultilineFlag);
     setSticky(flags & StickyFlag);
+    setUnicode(flags & UnicodeFlag);
 }
 
 void
@@ -455,6 +457,8 @@ RegExpObject::toString(JSContext* cx) const
         return nullptr;
     if (multiline() && !sb.append('m'))
         return nullptr;
+    if (unicode() && !sb.append('u'))
+        return nullptr;
     if (sticky() && !sb.append('y'))
         return nullptr;
 
@@ -515,7 +519,7 @@ RegExpShared::compile(JSContext* cx, HandleAtom pattern, HandleLinearString inpu
     /* Parse the pattern. */
     irregexp::RegExpCompileData data;
     if (!irregexp::ParsePattern(dummyTokenStream, cx->tempLifoAlloc(), pattern,
-                                multiline(), mode == MatchOnly, &data))
+                                multiline(), mode == MatchOnly, unicode(), &data))
     {
         return false;
     }
@@ -949,6 +953,10 @@ ParseRegExpFlags(const CharT* chars, size_t length, RegExpFlag* flagsOut, char16
             if (!HandleRegExpFlag(StickyFlag, flagsOut))
                 return false;
             break;
+          case 'u':
+            if (!HandleRegExpFlag(UnicodeFlag, flagsOut))
+                return false;
+            break;
           default:
             return false;
         }
diff --git a/js/src/vm/RegExpObject.h b/js/src/vm/RegExpObject.h
index 24e88deb5..dbf9efa98 100644
--- a/js/src/vm/RegExpObject.h
+++ b/js/src/vm/RegExpObject.h
@@ -51,9 +51,10 @@ enum RegExpFlag
     GlobalFlag      = 0x02,
     MultilineFlag   = 0x04,
     StickyFlag      = 0x08,
+    UnicodeFlag     = 0x10,
 
     NoFlags         = 0x00,
-    AllFlags        = 0x0f
+    AllFlags        = 0x1f
 };
 
 enum RegExpRunStatus
@@ -186,6 +187,7 @@ class RegExpShared
     bool global() const                 { return flags & GlobalFlag; }
     bool multiline() const              { return flags & MultilineFlag; }
     bool sticky() const                 { return flags & StickyFlag; }
+    bool unicode() const                { return flags & UnicodeFlag; }
 
     bool isCompiled(CompilationMode mode, bool latin1,
                     ForceByteCodeEnum force = DontForceByteCode) const {
@@ -340,9 +342,10 @@ class RegExpObject : public NativeObject
     static const unsigned IGNORE_CASE_FLAG_SLOT    = 3;
     static const unsigned MULTILINE_FLAG_SLOT      = 4;
     static const unsigned STICKY_FLAG_SLOT         = 5;
+    static const unsigned UNICODE_FLAG_SLOT        = 6;
 
   public:
-    static const unsigned RESERVED_SLOTS = 6;
+    static const unsigned RESERVED_SLOTS = 7;
     static const unsigned PRIVATE_SLOT = 7;
 
     static const Class class_;
@@ -407,6 +410,7 @@ class RegExpObject : public NativeObject
         flags |= ignoreCase() ? IgnoreCaseFlag : 0;
         flags |= multiline() ? MultilineFlag : 0;
         flags |= sticky() ? StickyFlag : 0;
+        flags |= unicode() ? UnicodeFlag : 0;
         return RegExpFlag(flags);
     }
 
@@ -432,10 +436,15 @@ class RegExpObject : public NativeObject
         setSlot(STICKY_FLAG_SLOT, BooleanValue(enabled));
     }
 
+    void setUnicode(bool enabled) {
+        setSlot(UNICODE_FLAG_SLOT, BooleanValue(enabled));
+    }
+
     bool ignoreCase() const { return getFixedSlot(IGNORE_CASE_FLAG_SLOT).toBoolean(); }
     bool global() const     { return getFixedSlot(GLOBAL_FLAG_SLOT).toBoolean(); }
     bool multiline() const  { return getFixedSlot(MULTILINE_FLAG_SLOT).toBoolean(); }
     bool sticky() const     { return getFixedSlot(STICKY_FLAG_SLOT).toBoolean(); }
+    bool unicode() const    { return getFixedSlot(UNICODE_FLAG_SLOT).toBoolean(); }
 
     bool getShared(JSContext* cx, RegExpGuard* g);
 
diff --git a/js/xpconnect/tests/chrome/test_xrayToJS.xul b/js/xpconnect/tests/chrome/test_xrayToJS.xul
index f600dd0ee..cbcd427d2 100644
--- a/js/xpconnect/tests/chrome/test_xrayToJS.xul
+++ b/js/xpconnect/tests/chrome/test_xrayToJS.xul
@@ -198,7 +198,7 @@ https://bugzilla.mozilla.org/show_bug.cgi?id=933681
 
   gPrototypeProperties['RegExp'] =
     ["constructor", "toSource", "toString", "compile", "exec", "test",
-     "flags", "global", "ignoreCase", "multiline", "source", "sticky",
+     "flags", "global", "ignoreCase", "multiline", "source", "sticky", "unicode",
      "lastIndex"];
 
   // Sort an array that may contain symbols as well as strings.
@@ -612,7 +612,7 @@ https://bugzilla.mozilla.org/show_bug.cgi?id=933681
 
       // Test with modified flags accessors
       iwin.eval(`
-var props = ["global", "ignoreCase", "multiline", "sticky", "source"];
+var props = ["global", "ignoreCase", "multiline", "sticky", "source", "unicode"];
 var origDescs = {};
 for (var prop of props) {
   origDescs[prop] = Object.getOwnPropertyDescriptor(RegExp.prototype, prop);