diff --git a/prism/config.yml b/prism/config.yml
index 67419af7c0..e395c5a8c3 100644
--- a/prism/config.yml
+++ b/prism/config.yml
@@ -234,6 +234,7 @@ warnings:
   - AMBIGUOUS_FIRST_ARGUMENT_MINUS
   - AMBIGUOUS_FIRST_ARGUMENT_PLUS
   - AMBIGUOUS_PREFIX_STAR
+  - AMBIGUOUS_PREFIX_STAR_STAR
   - AMBIGUOUS_SLASH
   - COMPARISON_AFTER_COMPARISON
   - DOT_DOT_DOT_EOL
diff --git a/prism/prism.c b/prism/prism.c
index 251ea9ac78..95ac872bb2 100644
--- a/prism/prism.c
+++ b/prism/prism.c
@@ -9610,7 +9610,10 @@ parser_lex(pm_parser_t *parser) {
 
                         pm_token_type_t type = PM_TOKEN_STAR_STAR;
 
-                        if (lex_state_spcarg_p(parser, space_seen) || lex_state_beg_p(parser)) {
+                        if (lex_state_spcarg_p(parser, space_seen)) {
+                            pm_parser_warn_token(parser, &parser->current, PM_WARN_AMBIGUOUS_PREFIX_STAR_STAR);
+                            type = PM_TOKEN_USTAR_STAR;
+                        } else if (lex_state_beg_p(parser)) {
                             type = PM_TOKEN_USTAR_STAR;
                         }
 
diff --git a/prism/templates/src/diagnostic.c.erb b/prism/templates/src/diagnostic.c.erb
index 23f89bfe63..8ca5cff3e8 100644
--- a/prism/templates/src/diagnostic.c.erb
+++ b/prism/templates/src/diagnostic.c.erb
@@ -314,6 +314,7 @@ static const pm_diagnostic_data_t diagnostic_messages[PM_DIAGNOSTIC_ID_MAX] = {
     [PM_WARN_AMBIGUOUS_FIRST_ARGUMENT_MINUS]    = { "ambiguous first argument; put parentheses or a space even after `-` operator", PM_WARNING_LEVEL_VERBOSE },
     [PM_WARN_AMBIGUOUS_FIRST_ARGUMENT_PLUS]     = { "ambiguous first argument; put parentheses or a space even after `+` operator", PM_WARNING_LEVEL_VERBOSE },
     [PM_WARN_AMBIGUOUS_PREFIX_STAR]             = { "ambiguous `*` has been interpreted as an argument prefix", PM_WARNING_LEVEL_VERBOSE },
+    [PM_WARN_AMBIGUOUS_PREFIX_STAR_STAR]        = { "ambiguous `**` has been interpreted as an argument prefix", PM_WARNING_LEVEL_VERBOSE },
     [PM_WARN_AMBIGUOUS_SLASH]                   = { "ambiguous `/`; wrap regexp in parentheses or add a space after `/` operator", PM_WARNING_LEVEL_VERBOSE },
     [PM_WARN_COMPARISON_AFTER_COMPARISON]       = { "comparison '%.*s' after comparison", PM_WARNING_LEVEL_VERBOSE },
     [PM_WARN_DOT_DOT_DOT_EOL]                   = { "... at EOL, should be parenthesized?", PM_WARNING_LEVEL_DEFAULT },