Pseudoaneurysms are potentially fatal complications of vascular trauma; however, they are rarely seen as the sole complication of penetrating injury. This article presents a case of a pseudoaneurysm with an associated arteriovenous fistula of the left lateral thoracic artery as a result of direct trauma from a knife stab. The patient presented only with a painful, swollen left pectoralis muscle. Upon diagnosis, he was taken to the interventional radiology suite and treated successfully with fluoroscopic guided coil embolization. Cases such as these are infrequent and should encourage more aggressive use of contrast-enhanced computed tomography imaging for soft tissue injury, as a missed traumatic pseudoaneurysm may result in life-threatening hemorrhage.