An anatomic study was undertaken to provide an understanding of the ligamentous disruption that occurs during acute dislocation of the thumb carpometacarpal joint. Thirty-eight cadaver thumbs were dissected free of soft tissue, with the ligaments preserved. The dorsoradial, posterior oblique, anterior oblique, and intermetacarpal ligaments were identified in all specimens. A dorsal dislocating force was applied to the base of the thumb metacarpal to recreate the mechanism of clinical dislocation. Serial sectioning of the ligaments was performed with the metacarpal in neutral, flexion, and extension. The primary restraint to dorsal dislocation was found to be the dorsoradial ligament, with the anterior oblique ligament allowing dislocation by subperiosteal stripping from the base of the first metacarpal. After reduction, the joint was most stable in pronation and extension, which tightened the anterior oblique ligament. We conclude that the dorsoradial ligament is the primary restraining force with respect to acute dorsal dislocation of the thumb carpometacarpal joint.