diff --git a/csharp/protos/unittest_issues.proto b/csharp/protos/unittest_issues.proto
index 1b5562f5c8..56dab2acd5 100644
--- a/csharp/protos/unittest_issues.proto
+++ b/csharp/protos/unittest_issues.proto
@@ -33,24 +33,25 @@ syntax = "proto3";
 // These proto descriptors have at one time been reported as an issue or defect.
 // They are kept here to replicate the issue, and continue to verify the fix.
 
-// Issue: Non-"Google.Protobuffers" namespace will ensure that protobuffer library types are qualified
-option csharp_namespace = "UnitTest.Issues.TestProtos";
-
+// Issue: Non-"Google.Protobuffers" namespace will ensure that protobuffer
+// library types are qualified
 package unittest_issues;
 
 import "google/protobuf/struct.proto";
 
+option csharp_namespace = "UnitTest.Issues.TestProtos";
+
 // Issue 307: when generating doubly-nested types, any references
 // should be of the form A.Types.B.Types.C.
 message Issue307 {
   message NestedOnce {
-    message NestedTwice {
-    }
+    message NestedTwice {}
   }
 }
 
-// Old issue 13: http://code.google.com/p/protobuf-csharp-port/issues/detail?id=13
-// New issue 309: https://github.com/protocolbuffers/protobuf/issues/309
+// Old issue 13:
+// http://code.google.com/p/protobuf-csharp-port/issues/detail?id=13 New issue
+// 309: https://github.com/protocolbuffers/protobuf/issues/309
 
 // message A {
 //    optional int32 _A = 1;
@@ -60,9 +61,9 @@ message Issue307 {
 //    optional int32 B_ = 1;
 // }
 
-//message AB {
-//    optional int32 a_b = 1;
-//}
+// message AB {
+//     optional int32 a_b = 1;
+// }
 
 // Similar issue with numeric names
 // Java code failed too, so probably best for this to be a restriction.
@@ -74,39 +75,40 @@ message Issue307 {
 // issue 19 - negative enum values
 
 enum NegativeEnum {
-    NEGATIVE_ENUM_ZERO = 0;
-    FiveBelow = -5;
-    MinusOne = -1;
+  NEGATIVE_ENUM_ZERO = 0;
+  FiveBelow = -5;
+  MinusOne = -1;
 }
 
 message NegativeEnumMessage {
-    NegativeEnum value = 1;
-    repeated NegativeEnum values = 2 [packed = false];
-    repeated NegativeEnum packed_values = 3 [packed=true];
+  NegativeEnum value = 1;
+  repeated NegativeEnum values = 2 [packed = false];
+  repeated NegativeEnum packed_values = 3 [packed = true];
 }
 
 // Issue 21: http://code.google.com/p/protobuf-csharp-port/issues/detail?id=21
 // Decorate fields with [deprecated=true] as [System.Obsolete]
 
 message DeprecatedChild {
-    option deprecated = true;
+  option deprecated = true;
 }
 
 enum DeprecatedEnum {
-    option deprecated = true;
-    DEPRECATED_ZERO = 0 [deprecated = true];
-    one = 1;
+  option deprecated = true;
+
+  DEPRECATED_ZERO = 0 [deprecated = true];
+  one = 1;
 }
 
 message DeprecatedFieldsMessage {
-    int32 PrimitiveValue = 1 [deprecated = true];
-    repeated int32 PrimitiveArray = 2 [deprecated = true];
+  int32 PrimitiveValue = 1 [deprecated = true];
+  repeated int32 PrimitiveArray = 2 [deprecated = true];
 
-    DeprecatedChild MessageValue = 3 [deprecated = true];
-    repeated DeprecatedChild MessageArray = 4 [deprecated = true];
+  DeprecatedChild MessageValue = 3 [deprecated = true];
+  repeated DeprecatedChild MessageArray = 4 [deprecated = true];
 
-    DeprecatedEnum EnumValue = 5 [deprecated = true];
-    repeated DeprecatedEnum EnumArray = 6 [deprecated = true];
+  DeprecatedEnum EnumValue = 5 [deprecated = true];
+  repeated DeprecatedEnum EnumArray = 6 [deprecated = true];
 }
 
 // Issue 45: http://code.google.com/p/protobuf-csharp-port/issues/detail?id=45
@@ -116,8 +118,7 @@ message ItemField {
 
 message ReservedNames {
   // Force a nested type called Types
-  message SomeNestedType {
-  }
+  message SomeNestedType {}
 
   int32 types = 1;
   int32 descriptor = 2;
@@ -148,7 +149,6 @@ message TestJsonFieldOrdering {
     int32 o2_int32 = 6;
     string o2_string = 3;
   }
-
 }
 
 message TestJsonName {
@@ -218,3 +218,9 @@ message DisambiguateCommonMembers {
   int32 on_construction = 11;
   int32 parser = 12;
 }
+
+message Issue11987Message {
+  int32 a = 1 [json_name = 'b'];
+  int32 b = 2 [json_name = 'a'];
+  int32 c = 3 [json_name = 'd'];
+}
diff --git a/csharp/src/Google.Protobuf.Test/JsonFormatterTest.cs b/csharp/src/Google.Protobuf.Test/JsonFormatterTest.cs
index f4dfde2435..963faedd4f 100644
--- a/csharp/src/Google.Protobuf.Test/JsonFormatterTest.cs
+++ b/csharp/src/Google.Protobuf.Test/JsonFormatterTest.cs
@@ -630,6 +630,18 @@ namespace Google.Protobuf
             Assert.AreEqual(expectedJson, JsonFormatter.Default.Format(populated));
         }
 
+        // See See https://github.com/protocolbuffers/protobuf/issues/11987
+        [Test]
+        public void JsonNamePriority()
+        {
+            // This tests both the formatter and the parser, but the issue was when parsing.
+            var original = new Issue11987Message { A = 10, B = 20, C = 30 };
+            var json = JsonFormatter.Default.Format(original);
+            AssertJson("{ 'b': 10, 'a': 20, 'd': 30 }", json);
+            var parsed = Issue11987Message.Parser.ParseJson(json);
+            Assert.AreEqual(original, parsed);
+        }
+
         // Sanity tests for WriteValue. Not particularly comprehensive, as it's all covered above already,
         // as FormatMessage uses WriteValue.
 
diff --git a/csharp/src/Google.Protobuf/Reflection/MessageDescriptor.cs b/csharp/src/Google.Protobuf/Reflection/MessageDescriptor.cs
index f2bb61c2cd..d9944f6026 100644
--- a/csharp/src/Google.Protobuf/Reflection/MessageDescriptor.cs
+++ b/csharp/src/Google.Protobuf/Reflection/MessageDescriptor.cs
@@ -117,9 +117,15 @@ namespace Google.Protobuf.Reflection
         private static ReadOnlyDictionary<string, FieldDescriptor> CreateJsonFieldMap(IList<FieldDescriptor> fields)
         {
             var map = new Dictionary<string, FieldDescriptor>();
+            // The ordering is important here: JsonName takes priority over Name,
+            // which means we need to put JsonName values in the map after *all*
+            // Name keys have been added. See https://github.com/protocolbuffers/protobuf/issues/11987
             foreach (var field in fields)
             {
                 map[field.Name] = field;
+            }
+            foreach (var field in fields)
+            {
                 map[field.JsonName] = field;
             }
             return new ReadOnlyDictionary<string, FieldDescriptor>(map);