Posterior unidirectional instability of the shoulder following trauma is frequently associated with a cartilage-bone defect on the anterior joint surface of the humeral head. This defect can catch on the posterior joint surface during internal rotation of the arm, which causes redislocation of the humeral head in a posterior direction, thus increasing joint destruction. We describe a new arthroscopic surgical technique in which the subscapularis tendon is mobilized and attached to the defect using nonabsorbable or bioabsorbable suture anchors. In an manner analogous to open surgery with arthrotomy and transposition of the subscapularis tendon insertion into the defect, this arthroscopic technique fills up the defect, blocks extension during internal rotation of the arm, and safely avoids redislocation of the humeral head in a dorsal direction. The arthroscopic technique we describe is easy and safe to perform. Complicated tightening of the posterior joint capsule is no longer necessary following acute injury.