diff --git a/encoding/protojson/decode.go b/encoding/protojson/decode.go
index 9bf4e8c..5ba9ebf 100644
--- a/encoding/protojson/decode.go
+++ b/encoding/protojson/decode.go
@@ -170,7 +170,7 @@
 		if strings.HasPrefix(name, "[") && strings.HasSuffix(name, "]") {
 			// Only extension names are in [name] format.
 			extName := pref.FullName(name[1 : len(name)-1])
-			extType, err := d.findExtension(extName)
+			extType, err := d.opts.Resolver.FindExtensionByName(extName)
 			if err != nil && err != protoregistry.NotFound {
 				return d.newError(tok.Pos(), "unable to resolve %s: %v", tok.RawString(), err)
 			}
@@ -257,15 +257,6 @@
 	}
 }
 
-// findExtension returns protoreflect.ExtensionType from the resolver if found.
-func (d decoder) findExtension(xtName pref.FullName) (pref.ExtensionType, error) {
-	xt, err := d.opts.Resolver.FindExtensionByName(xtName)
-	if err == nil {
-		return xt, nil
-	}
-	return messageset.FindMessageSetExtension(d.opts.Resolver, xtName)
-}
-
 func isKnownValue(fd pref.FieldDescriptor) bool {
 	md := fd.Message()
 	return md != nil && md.FullName() == genid.Value_message_fullname
diff --git a/encoding/protojson/decode_test.go b/encoding/protojson/decode_test.go
index 5e3dcff..4791f65 100644
--- a/encoding/protojson/decode_test.go
+++ b/encoding/protojson/decode_test.go
@@ -1403,7 +1403,7 @@
     "optString": "not a messageset extension"
   }
 }`,
-		wantErr: `unknown field "[pb2.FakeMessageSetExtension]"`,
+		wantErr: `unable to resolve "[pb2.FakeMessageSetExtension]": found wrong type`,
 		skip:    !flags.ProtoLegacy,
 	}, {
 		desc:         "not real MessageSet 3",
diff --git a/encoding/prototext/decode.go b/encoding/prototext/decode.go
index cab95a4..8cce1e0 100644
--- a/encoding/prototext/decode.go
+++ b/encoding/prototext/decode.go
@@ -172,7 +172,7 @@
 
 		case text.TypeName:
 			// Handle extensions only. This code path is not for Any.
-			xt, xtErr = d.findExtension(pref.FullName(tok.TypeName()))
+			xt, xtErr = d.opts.Resolver.FindExtensionByName(pref.FullName(tok.TypeName()))
 
 		case text.FieldNumber:
 			isFieldNumberName = true
@@ -269,15 +269,6 @@
 	return nil
 }
 
-// findExtension returns protoreflect.ExtensionType from the Resolver if found.
-func (d decoder) findExtension(xtName pref.FullName) (pref.ExtensionType, error) {
-	xt, err := d.opts.Resolver.FindExtensionByName(xtName)
-	if err == nil {
-		return xt, nil
-	}
-	return messageset.FindMessageSetExtension(d.opts.Resolver, xtName)
-}
-
 // unmarshalSingular unmarshals a non-repeated field value specified by the
 // given FieldDescriptor.
 func (d decoder) unmarshalSingular(fd pref.FieldDescriptor, m pref.Message) error {
diff --git a/encoding/prototext/decode_test.go b/encoding/prototext/decode_test.go
index dceded1..441dcb2 100644
--- a/encoding/prototext/decode_test.go
+++ b/encoding/prototext/decode_test.go
@@ -1508,7 +1508,7 @@
   opt_string: "not a messageset extension"
 }
 `,
-		wantErr: "unknown field: [pb2.FakeMessageSetExtension]",
+		wantErr: `unable to resolve [[pb2.FakeMessageSetExtension]]: found wrong type`,
 		skip:    !flags.ProtoLegacy,
 	}, {
 		desc:         "not real MessageSet 3",
diff --git a/internal/encoding/messageset/messageset.go b/internal/encoding/messageset/messageset.go
index b1eeea5..453a81a 100644
--- a/internal/encoding/messageset/messageset.go
+++ b/internal/encoding/messageset/messageset.go
@@ -11,7 +11,6 @@
 	"google.golang.org/protobuf/encoding/protowire"
 	"google.golang.org/protobuf/internal/errors"
 	pref "google.golang.org/protobuf/reflect/protoreflect"
-	preg "google.golang.org/protobuf/reflect/protoregistry"
 )
 
 // The MessageSet wire format is equivalent to a message defiend as follows,
@@ -48,33 +47,17 @@
 	return ok && xmd.IsMessageSet()
 }
 
-// IsMessageSetExtension reports this field extends a MessageSet.
+// IsMessageSetExtension reports this field properly extends a MessageSet.
 func IsMessageSetExtension(fd pref.FieldDescriptor) bool {
-	if fd.Name() != ExtensionName {
+	switch {
+	case fd.Name() != ExtensionName:
+		return false
+	case !IsMessageSet(fd.ContainingMessage()):
+		return false
+	case fd.FullName().Parent() != fd.Message().FullName():
 		return false
 	}
-	if fd.FullName().Parent() != fd.Message().FullName() {
-		return false
-	}
-	return IsMessageSet(fd.ContainingMessage())
-}
-
-// FindMessageSetExtension locates a MessageSet extension field by name.
-// In text and JSON formats, the extension name used is the message itself.
-// The extension field name is derived by appending ExtensionName.
-func FindMessageSetExtension(r preg.ExtensionTypeResolver, s pref.FullName) (pref.ExtensionType, error) {
-	name := s.Append(ExtensionName)
-	xt, err := r.FindExtensionByName(name)
-	if err != nil {
-		if err == preg.NotFound {
-			return nil, err
-		}
-		return nil, errors.Wrap(err, "%q", name)
-	}
-	if !IsMessageSetExtension(xt.TypeDescriptor()) {
-		return nil, preg.NotFound
-	}
-	return xt, nil
+	return true
 }
 
 // SizeField returns the size of a MessageSet item field containing an extension
diff --git a/reflect/protoregistry/registry.go b/reflect/protoregistry/registry.go
index 5e5f967..7dcf4d9 100644
--- a/reflect/protoregistry/registry.go
+++ b/reflect/protoregistry/registry.go
@@ -21,7 +21,9 @@
 	"strings"
 	"sync"
 
+	"google.golang.org/protobuf/internal/encoding/messageset"
 	"google.golang.org/protobuf/internal/errors"
+	"google.golang.org/protobuf/internal/flags"
 	"google.golang.org/protobuf/reflect/protoreflect"
 )
 
@@ -613,6 +615,26 @@
 		if xt, _ := v.(protoreflect.ExtensionType); xt != nil {
 			return xt, nil
 		}
+
+		// MessageSet extensions are special in that the name of the extension
+		// is the name of the message type used to extend the MessageSet.
+		// This naming scheme is used by text and JSON serialization.
+		//
+		// This feature is protected by the ProtoLegacy flag since MessageSets
+		// are a proto1 feature that is long deprecated.
+		if flags.ProtoLegacy {
+			if _, ok := v.(protoreflect.MessageType); ok {
+				field := field.Append(messageset.ExtensionName)
+				if v := r.typesByName[field]; v != nil {
+					if xt, _ := v.(protoreflect.ExtensionType); xt != nil {
+						if messageset.IsMessageSetExtension(xt.TypeDescriptor()) {
+							return xt, nil
+						}
+					}
+				}
+			}
+		}
+
 		return nil, errors.New("found wrong type: got %v, want extension", typeName(v))
 	}
 	return nil, NotFound