The os trigonum syndrome refers to symptoms produced by pathology of the lateral tubercle of the posterior talar process. Pain can be caused by disruption of the cartilaginous synchondrosis between the os trigonum and the lateral talar tubercle as a result of repetitive microtrauma and chronic inflammation. Additional etiologies include trigonal process fracture, flexor hallucis longus tenosynovitis, posterior tibiotalar impingement by bone block, and intraarticular loose bodies. This pictorial essay explores the role of imaging modalities in the diagnosis and treatment of the os trigonum syndrome, a symptom complex that may present difficult diagnostic problems.