diff --git a/test/yarp/errors_test.rb b/test/yarp/errors_test.rb
index 54cbda14d1..41cb6b7f47 100644
--- a/test/yarp/errors_test.rb
+++ b/test/yarp/errors_test.rb
@@ -1255,6 +1255,20 @@ module YARP
       assert_error_messages "0x1_1_", error_messages
     end
 
+    def test_alnum_delimiters
+      error_messages = ["Invalid `%` token"]
+
+      assert_error_messages "%qXfooX", error_messages
+      assert_error_messages "%QXfooX", error_messages
+      assert_error_messages "%wXfooX", error_messages
+      assert_error_messages "%WxfooX", error_messages
+      assert_error_messages "%iXfooX", error_messages
+      assert_error_messages "%IXfooX", error_messages
+      assert_error_messages "%xXfooX", error_messages
+      assert_error_messages "%rXfooX", error_messages
+      assert_error_messages "%sXfooX", error_messages
+    end
+
     private
 
     def assert_errors(expected, source, errors, compare_ripper: RUBY_ENGINE == "ruby")
diff --git a/yarp/yarp.c b/yarp/yarp.c
index b343566ee0..7c4b347678 100644
--- a/yarp/yarp.c
+++ b/yarp/yarp.c
@@ -7077,9 +7077,10 @@ parser_lex(yp_parser_t *parser) {
 
                 // % %= %i %I %q %Q %w %W
                 case '%': {
-                    // If there is no subsequent character then we have an invalid token. We're
-                    // going to say it's the percent operator because we don't want to move into the
-                    // string lex mode unnecessarily.
+                    // If there is no subsequent character then we have an
+                    // invalid token. We're going to say it's the percent
+                    // operator because we don't want to move into the string
+                    // lex mode unnecessarily.
                     if ((lex_state_beg_p(parser) || lex_state_arg_p(parser)) && (parser->current.end >= parser->end)) {
                         yp_diagnostic_list_append(&parser->error_list, parser->current.start, parser->current.end, YP_ERR_INVALID_PERCENT);
                         LEX(YP_TOKEN_PERCENT);
@@ -7110,6 +7111,14 @@ parser_lex(yp_parser_t *parser) {
                             }
                         }
 
+                        // Delimiters for %-literals cannot be alphanumeric. We
+                        // validate that here.
+                        uint8_t delimiter = peek_offset(parser, 1);
+                        if (delimiter >= 0x80 || parser->encoding.alnum_char(&delimiter, 1)) {
+                            yp_diagnostic_list_append(&parser->error_list, parser->current.start, parser->current.end, YP_ERR_INVALID_PERCENT);
+                            goto lex_next_token;
+                        }
+
                         switch (peek(parser)) {
                             case 'i': {
                                 parser->current.end++;