Background: Autograft palmaris has been the primary choice for the reconstruction of the medial ulnar collateral ligament (UCL) in the elbow. Agenesis of the palmaris tendon is not rare, and outcomes of allograft reconstruction in the breadth of athlete types found in the adolescent population are lacking. Hypothesis: Allograft tendon reconstruction of the medial UCL in the young elbow would have low failure rates and satisfactory outcome scores. Study Design: Case series; Level of evidence, 4. Methods: The records of patients who underwent allograft medial UCL reconstruction (UCLR) by a single surgeon between 2009 and 2019 were reviewed. Patient-reported outcome scores in adolescent patients obtained at a minimum 4-year follow-up (for intermediate assessment) were recorded, with no exclusion according to sex or sport type. Outcomes included the Timmerman-Andrews score, the Single Assessment Numeric Evaluation (SANE), and the Conway-Jobe score. Results: Of 29 patients who underwent allograft UCLR, 10 adolescents (40% women; mean age at surgery, 15.8 years [age range, 15-17 years]) met the inclusion criteria and were included in the study. The mean follow-up was 8 years (range, 4.3-11.9 years). There were 4 overhead athletes (baseball, water polo) and 6 impact sports athletes (motocross, gymnastics, wrestling, and soccer). The mean SANE score was 86.3, and the mean subjective Timmerman-Andrews score was 92.5. The Conway-Jobe score was “excellent” in 7 of 10 participants; nonetheless, 3 elected not to return to sport (2 for reasons unrelated to the elbow). No patients experienced loss of range of motion, contracture, or ulnar nerve neuropathy. There was 1 patient with early failure (10%) who required revision reconstruction . Conclusion: Allograft reconstruction for medial UCL instability in adolescent patients from sport and trauma mechanisms demonstrated excellent patient-reported functional scores in this study. If the patient and surgeon desire to avoid autograft morbidity or agenesis of the palmaris longus, allograft tendon UCLR appears viable for both the throwing and the high-impact adolescent athlete, regardless of sex.