Thumb metacarpophalangeal (MCP) joint dislocations are infrequent injuries, and palmar dislocations rarely occur. The following is a case report of a complex palmar dislocation of the thumb MCP joint that demonstrates the unique physical finding of paradoxical flexion of the MCP joint (and interphalangeal joint extension) in conjunction with the patient's active attempt to extend the MCP joint. This finding proved to be indicative of extensor pollicis tendon displacement palmar to the MCP joint. This soft-tissue incarceration necessitated open reduction. A careful clinical examination is essential for an accurate evaluation of these injuries. Thus, a satisfactory clinical outcome can be attained with strict attention to the intraoperative pathology and technique of repair.