Peripheral arterial pseudoaneurysm is an uncommon complication after open reduction and internal fixation of bone fractures. Here, we describe a rare case of a 72-year-old man who developed a delayed pseudoaneurysm of the anterior interosseous artery after open reduction and internal fixation of an isolated ulnar fracture (nightstick fracture). He presented with a painful forearm mass and anterior interosseous nerve (AIN) neuropathy. Though rare, the appearance of AIN neuropathy is a potential complication of forearm fractures' fixation that should be diagnosed early and managed with a prompt and appropriate treatment.