diff --git a/prism/config.yml b/prism/config.yml
index e395c5a8c3..f7dcc1f79f 100644
--- a/prism/config.yml
+++ b/prism/config.yml
@@ -233,6 +233,7 @@ errors:
 warnings:
   - AMBIGUOUS_FIRST_ARGUMENT_MINUS
   - AMBIGUOUS_FIRST_ARGUMENT_PLUS
+  - AMBIGUOUS_PREFIX_AMPERSAND
   - AMBIGUOUS_PREFIX_STAR
   - AMBIGUOUS_PREFIX_STAR_STAR
   - AMBIGUOUS_SLASH
diff --git a/prism/prism.c b/prism/prism.c
index 95ac872bb2..947d1a92bb 100644
--- a/prism/prism.c
+++ b/prism/prism.c
@@ -9912,7 +9912,10 @@ parser_lex(pm_parser_t *parser) {
                     }
 
                     pm_token_type_t type = PM_TOKEN_AMPERSAND;
-                    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_AMPERSAND);
+                        type = PM_TOKEN_UAMPERSAND;
+                    } else if (lex_state_beg_p(parser)) {
                         type = PM_TOKEN_UAMPERSAND;
                     }
 
diff --git a/prism/templates/src/diagnostic.c.erb b/prism/templates/src/diagnostic.c.erb
index 8ca5cff3e8..5ba4d62317 100644
--- a/prism/templates/src/diagnostic.c.erb
+++ b/prism/templates/src/diagnostic.c.erb
@@ -313,6 +313,7 @@ static const pm_diagnostic_data_t diagnostic_messages[PM_DIAGNOSTIC_ID_MAX] = {
     // Warnings
     [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_AMPERSAND]        = { "ambiguous `&` has been interpreted as an argument prefix", 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 },