Merkel cell carcinoma (MCC) is a rare and aggressive cutaneous malignancy with a high propensity for local recurrence and regional and distant metastases. The main treatment is surgery with narrow excision margins and draining nodes, plus or minus adjuvant radiotherapy (RT) on the surgical bed and/or lymph nodes. We performed a systematic review and meta-analysis of the benefits of adjuvant RT in MCC treatment. PubMed, EMBASE, and the Cochrane Library were systematically searched to identify relevant studies published before September 2018. Prospective trials and retrospective series comparing adjuvant RT vs. no RT in resected primary MCCs were included. Primary endpoint was to evaluate the outcomes of MCC patients who received adjuvant RT in term of overall survival (OS) and disease-free survival (DFS). Hazard ratios (HRs) for OS and DFS were aggregated according to a fixed or random effect model. Secondary endpoints were local, locoregional, and distant DFS. A total of 17,179 MCCs across 29 studies were analysed. There was a significant difference in OS between the RT and no RT arms (HR = 0.81, 95%CI 0.75-0.86, P < 0.001). There was also a significant difference in DFS in favour of adjuvant RT (HR = 0.45, 95%CI 0.32-0.62, P < 0.001). Adjuvant RT improved locoregional DFS and local DFS but not distant DFS (HR = 0.3, 95%CI 0.22-0.42; HR = 0.21, 95%CI 0.14-0.33, and HR = 0.79, 95%CI 0.49-1.14, respectively). Meta-regression analysis showed that high Newcastle-Ottawa scale scores, stage I-II MCCs, shorter follow-up durations, size >2 cm, and being of a younger age were associated with increased OS. This systematic review and meta-analysis suggests a survival and DFS benefit for postoperative radiation of MCCs. Intermediate stage MCCs derive the maximum benefit with local and regional relapses reduced by 80% and 70%, respectively. Conversely, distant metastases were not significantly prevented.