diff --git a/js/src/builtin/Eval.cpp b/js/src/builtin/Eval.cpp
index cb6197995bb1..482cb4dd22f2 100644
--- a/js/src/builtin/Eval.cpp
+++ b/js/src/builtin/Eval.cpp
@@ -175,7 +175,8 @@ static EvalJSONResult ParseEvalStringAsJSON(
                                            chars.begin().get() + 1U, len - 2);
 
   Rooted<JSONParser<CharT>> parser(
-      cx, JSONParser<CharT>(cx, jsonChars, JSONParserBase::NoError));
+      cx, JSONParser<CharT>(cx, jsonChars,
+                            JSONParserBase::ParseType::AttemptForEval));
   if (!parser.parse(rval)) {
     return EvalJSON_Failure;
   }
diff --git a/js/src/builtin/JSON.cpp b/js/src/builtin/JSON.cpp
index 34f308f252bb..d24654b92f88 100644
--- a/js/src/builtin/JSON.cpp
+++ b/js/src/builtin/JSON.cpp
@@ -1008,7 +1008,8 @@ bool js::ParseJSONWithReviver(JSContext* cx,
                               const mozilla::Range<const CharT> chars,
                               HandleValue reviver, MutableHandleValue vp) {
   /* 15.12.2 steps 2-3. */
-  Rooted<JSONParser<CharT>> parser(cx, JSONParser<CharT>(cx, chars));
+  Rooted<JSONParser<CharT>> parser(
+      cx, JSONParser<CharT>(cx, chars, JSONParserBase::ParseType::JSONParse));
   if (!parser.parse(vp)) {
     return false;
   }
diff --git a/js/src/jit-test/tests/basic/eval-json-differences.js b/js/src/jit-test/tests/basic/eval-json-differences.js
new file mode 100644
index 000000000000..eb7ff3389b41
--- /dev/null
+++ b/js/src/jit-test/tests/basic/eval-json-differences.js
@@ -0,0 +1,25 @@
+load(libdir + "asserts.js");
+
+// eval({__proto__}) changes [[Prototype]]
+var obj = eval('({"__proto__": null})');
+assertEq(Object.getPrototypeOf(obj), null);
+assertEq(Object.prototype.hasOwnProperty.call(obj, "__proto__"), false);
+
+// JSON.parse({__proto__}) creates new property __proto__
+obj = JSON.parse('{"__proto__": null}');
+assertEq(Object.getPrototypeOf(obj), Object.prototype);
+assertEq(Object.prototype.hasOwnProperty.call(obj, "__proto__"), true);
+
+// If __proto__ appears more than once as quoted or unquoted property name in an
+// object initializer expression, that's an error.
+//(No other property name has this restriction.)"
+assertThrowsInstanceOf(() =>
+    eval('({ "__proto__" : null, "__proto__" : null })'), SyntaxError);
+
+assertThrowsInstanceOf(() =>
+    eval('  ({ "__proto__" : null, "__proto__" : null })'), SyntaxError);
+
+// JSON.parse doesn't care about duplication, the last definition counts.
+obj = JSON.parse('{"__proto__": null, "__proto__": 5}');
+assertEq(Object.getPrototypeOf(obj), Object.prototype);
+assertEq(obj["__proto__"], 5);
diff --git a/js/src/vm/JSONParser.cpp b/js/src/vm/JSONParser.cpp
index 3641e94a10a6..79210f352ac9 100644
--- a/js/src/vm/JSONParser.cpp
+++ b/js/src/vm/JSONParser.cpp
@@ -77,7 +77,7 @@ void JSONParser<CharT>::getTextPosition(uint32_t* column, uint32_t* line) {
 
 template <typename CharT>
 void JSONParser<CharT>::error(const char* msg) {
-  if (errorHandling == RaiseError) {
+  if (parseType == ParseType::JSONParse) {
     uint32_t column = 1, line = 1;
     getTextPosition(&column, &line);
 
@@ -93,7 +93,9 @@ void JSONParser<CharT>::error(const char* msg) {
   }
 }
 
-bool JSONParserBase::errorReturn() { return errorHandling == NoError; }
+bool JSONParserBase::errorReturn() {
+  return parseType == ParseType::AttemptForEval;
+}
 
 template <typename CharT>
 template <JSONParserBase::StringType ST>
@@ -666,6 +668,17 @@ bool JSONParser<CharT>::parse(MutableHandleValue vp) {
       JSONMember:
         if (token == String) {
           jsid id = AtomToId(atomValue());
+          if (parseType == ParseType::AttemptForEval) {
+            // In |JSON.parse|, "__proto__" is a property like any other and may
+            // appear multiple times. In object literal syntax, "__proto__" is
+            // prototype mutation and can appear at most once. |JSONParser| only
+            // supports the former semantics, so if this parse attempt is for
+            // |eval|, return true (without reporting an error) to indicate the
+            // JSON parse attempt was unsuccessful.
+            if (id == NameToId(cx->names().proto)) {
+              return true;
+            }
+          }
           PropertyVector& properties = stack.back().properties();
           if (!properties.append(IdValuePair(id))) {
             return false;
diff --git a/js/src/vm/JSONParser.h b/js/src/vm/JSONParser.h
index 2f638fc96341..eb527f8ff886 100644
--- a/js/src/vm/JSONParser.h
+++ b/js/src/vm/JSONParser.h
@@ -22,7 +22,14 @@ namespace js {
 // can be shared between the two encodings.
 class MOZ_STACK_CLASS JSONParserBase {
  public:
-  enum ErrorHandling { RaiseError, NoError };
+  enum class ParseType {
+    // Parsing a string as if by JSON.parse.
+    JSONParse,
+    // Parsing what may or may not be JSON in a string of eval code.
+    // In this case, a failure to parse indicates either syntax that isn't JSON,
+    // or syntax that has different semantics in eval code than in JSON.
+    AttemptForEval,
+  };
 
  private:
   /* Data members */
@@ -31,7 +38,7 @@ class MOZ_STACK_CLASS JSONParserBase {
  protected:
   JSContext* const cx;
 
-  const ErrorHandling errorHandling;
+  const ParseType parseType;
 
   enum Token {
     String,
@@ -114,9 +121,9 @@ class MOZ_STACK_CLASS JSONParserBase {
   Token lastToken;
 #endif
 
-  JSONParserBase(JSContext* cx, ErrorHandling errorHandling)
+  JSONParserBase(JSContext* cx, ParseType parseType)
       : cx(cx),
-        errorHandling(errorHandling),
+        parseType(parseType),
         stack(cx),
         freeElements(cx),
         freeProperties(cx)
@@ -132,7 +139,7 @@ class MOZ_STACK_CLASS JSONParserBase {
   JSONParserBase(JSONParserBase&& other)
       : v(other.v),
         cx(other.cx),
-        errorHandling(other.errorHandling),
+        parseType(other.parseType),
         stack(std::move(other.stack)),
         freeElements(std::move(other.freeElements)),
         freeProperties(std::move(other.freeProperties))
@@ -212,8 +219,8 @@ class MOZ_STACK_CLASS JSONParser : public JSONParserBase {
 
   /* Create a parser for the provided JSON data. */
   JSONParser(JSContext* cx, mozilla::Range<const CharT> data,
-             ErrorHandling errorHandling = RaiseError)
-      : JSONParserBase(cx, errorHandling),
+             ParseType parseType)
+      : JSONParserBase(cx, parseType),
         current(data.begin()),
         begin(current),
         end(data.end()) {