BACKGROUND: The advent of modular shoulder arthroplasty systems has allowed the conversion of hemiarthroplasty or total shoulder arthroplasty to reverse total shoulder arthroplasty (RTSA) without removing a well-fixed stem. QUESTIONS/PURPOSES: To determine the feasibility, functional outcome, and complication profile of RTSA modular conversion. METHODS: A prospective shoulder arthroplasty registry was queried for consecutive patients scheduled for a modular conversion from January 1, 2007, to April 1, 2015. Eligible patients had medical charts and operative records reviewed for preoperative diagnosis, age, medical comorbidities, preoperative American Shoulder and Elbow Society (ASES) score, preoperative Visual Analogue Scale (VAS) pain and instability scores, and intraoperative findings. Each patient was then contacted by telephone or mail to complete up-to-date ASES and VAS questionnaires. RESULTS: Seventeen patients underwent a modular conversion. Nine patients were scheduled for modular conversion but underwent humeral revision due to excessive soft tissue tension (65.3% modular conversion rate). Average follow-up was 37.4 months (range 10.0-67.6 months). Pain scores improved from 5.3 (range 0.4 to 8.0) to 2.4 (range 0 to 9.3) (p < 0.01), instability VAS from 5.2 (range 0 to 10) to 1.1 (range 0 to 6.8) (p < 0.01), and ASES scores improved from 35.2 (range 20.7 to 61.3) to 65.6 (range 11.8 to 92) (p < 0.01). CONCLUSIONS: Modular conversion of an anatomic to a RTSA is feasible in a majority of patients. Despite the complexity of the procedure, modular conversion of hemiarthroplasty or TSA to RTSA can significantly improve functional outcomes with a low rate of complications.