Lateral ulnar collateral ligament (LUCL) reconstruction is a commonly used surgical procedure to treat posterolateral rotatory instability (PLRI) of the elbow. We report a 17-year-old right-handed pitcher with symptomatic PLRI after an injury to his LUCL during a baseball game. Having failed 8 months of conservative treatment, a LUCL reconstruction using the modified O'Driscoll technique was performed with the contralateral gracilis autograft. At 5-year follow-up, an excellent patient-reported outcome was achieved and the patient continued to play baseball at the recreational level. This type of injury is rarely reported in adolescent baseball players, and improving our knowledge of the long-term outcomes is crucial to assist clinicians in counseling patients about expectations after surgery.