Objective: To study the effects of supervised training in adults with subacromial pain syndrome. Data Sources: Embase, MEDLINE, Cochrane Library, Cumulative Index to Nursing and Allied Health, and Physiotherapy Evidence Database were searched from inception to March 2020. Study Selection: Independent reviewers selected randomized controlled trials comparing supervised training with (1) no training or (2) self-training in adults with subacromial pain syndrome lasting for at least 1 month. Critical outcomes were shoulder pain, function, and patient-perceived effect. Important outcomes included other potential benefits and adverse events at 3-month follow-up. Data Extraction: Two independent reviewers extracted data for the meta-analysis. Risk of bias was assessed using the Cochrane Risk of Bias tool 1, and certainty of evidence was evaluated using the Grades of Recommendation Assessment, Development, and Evaluation (GRADE). Data Synthesis: Ten studies (n=597, 43% female) were included. Supervised training resulted in larger improvements than no training on pain (at rest: n=286; mean difference [MD], 1.68; 95% confidence interval [CI], 0.31-3.06 on 0-10 scale; during movement: n=353; MD, 1.84; 95% CI,0.91-2.76), function (n=396; standardized MD, 0.30; 95% CI, 0.07-0.52), and patient-perceived effect (n=118; risk ratio, 1.43; 95% CI, 0.87-2.34). Supervised training had potential benefits regarding quality of life, return to work, dropout, and training adherence, albeit more patients reported mild, transient pain after training. Supervised training and self-training showed equal improvements on pain (n=44) and function (n=76), with no data describing patient-perceived effect. Certainty of evidence was low for critical outcomes and low-moderate for other outcomes. Conclusions: Supervised training might be superior to no training and equally effective as self-training on critical and important outcomes. Based on low-moderate certainty of evidence, these findings support a weak recommendation for supervised training in adults with subacromial pain syndrome.