diff --git a/.changeset/fix-sequence-hang-missing-space-after-as.md b/.changeset/fix-sequence-hang-missing-space-after-as.md
new file mode 100644
index 00000000000..505cc9a0fed
--- /dev/null
+++ b/.changeset/fix-sequence-hang-missing-space-after-as.md
@@ -0,0 +1,5 @@
+---
+'mermaid': patch
+---
+
+fix: prevent sequence diagram hang when "as" is used without a trailing space in participant declarations
diff --git a/packages/mermaid/src/diagrams/sequence/parser/sequenceDiagram.jison b/packages/mermaid/src/diagrams/sequence/parser/sequenceDiagram.jison
index 49b8daba61b..a5cdd49f383 100644
--- a/packages/mermaid/src/diagrams/sequence/parser/sequenceDiagram.jison
+++ b/packages/mermaid/src/diagrams/sequence/parser/sequenceDiagram.jison
@@ -36,6 +36,7 @@
 <ID>[^<>:\n,;@\s]+(?=\s+as\s)                                   { yytext = yytext.trim(); this.begin('ALIAS'); return 'ACTOR'; }
 <ID>[^<>:\n,;@]+(?=\s*[\n;#]|$)                                 { yytext = yytext.trim(); this.popState(); return 'ACTOR'; }
 <ID>[^<>:\n,;@]*\<[^\n]*                                        { this.popState(); return 'INVALID'; }
+<ID>[^\n]+                                                      { yytext = yytext.trim(); this.popState(); return 'INVALID'; }
 "box"															{ this.begin('LINE'); return 'box'; }
 "participant"                                                   { this.begin('ID'); return 'participant'; }
 "actor"                                                   		{ this.begin('ID'); return 'participant_actor'; }
diff --git a/packages/mermaid/src/diagrams/sequence/sequenceDiagram.spec.js b/packages/mermaid/src/diagrams/sequence/sequenceDiagram.spec.js
index 7b75f1d4357..e6c664dc8bd 100644
--- a/packages/mermaid/src/diagrams/sequence/sequenceDiagram.spec.js
+++ b/packages/mermaid/src/diagrams/sequence/sequenceDiagram.spec.js
@@ -2622,6 +2622,20 @@ Bob->>Alice:Got it!
       expect(error).toBe(true);
     });
 
+    it('should not hang when "as" is used without a space before the alias text', async () => {
+      let errorMessage = '';
+      try {
+        await Diagram.fromText(`
+          sequenceDiagram
+          participant X_AutoPublishable asAAAAAAAAAAAAA:AAAAAAAAAAAAA
+        `);
+      } catch (e) {
+        errorMessage = e instanceof Error ? e.message : String(e);
+      }
+      expect(errorMessage).not.toBe('');
+      expect(errorMessage).not.toContain('Lexical error');
+    }, 5000);
+
     it('should parse participant with stereotype and alias', async () => {
       const diagram = await Diagram.fromText(`
       sequenceDiagram