Background: Rehabilitation contributes to post-operative success following reverse total shoulder arthroplasty; however, randomised trials comparing the effectiveness of rehabilitation following reverse total shoulder arthroplasty are lacking. This study sought to determine if early, active mobilisation targeting the deltoid and the external rotator muscles, would exhibit greater improvements in post-operative outcomes compared to a delayed and deltoid-focused mobilisation programme. Methods: Patients scheduled for reverse total shoulder arthroplasty were randomly assigned to either an early active or delayed active rehabilitation group. Patient-reported outcomes for pain and function were assessed pre-surgery and at 3, 6 and 12 months post-surgery. Objective measures (Constant Score, range of motion, isometric strength) were assessed at 3, 6 and 12 months post-surgery. Results: Sixty-one patients (63 shoulders) underwent reverse total shoulder arthroplasty. There were no significant interaction effects or between-group differences for any patient-reported outcomes or objective measures at 3, 6 or 12 months post-surgery. However, significantly better (p = 0.019) active arm flexion was observed in the early active group at three months post-surgery. Significantly more patients in the early active group reported improvement in patient-reported function that reached minimal clinically important difference from three to six months post-surgery (p = 0.016). Conclusion: Early, active rehabilitation after reverse total shoulder arthroplasty is safe and effective, and may have early clinical benefits over a conservative, delayed mobilisation programme. Level of evidence: Therapy, level 1b. Trial registered 15 June 2016 at www.anzctr.org.au (ACTRN12616000779471).