Despite surgical resection, many patients with muscle invasive urothelial carcinoma (MIUC) experience recurrence. Adjuvant immune checkpoint inhibition (ICI) following radical resection in patients with MIUC demonstrates disparate outcomes among phase III randomized controlled trials (RCTs). Our objective was to synthesize available data regarding the disease-free survival (DFS) benefit of adjuvant ICIs for patients with MIUC and evaluate the overall safety profile of ICIs in this setting. The protocol was registered with PROSPERO, CRD42022352587. We searched MEDLINE, Embase, CENTRAL, and relevant conference proceedings from inception up to January 29, 2024. Only phase III RCTs comparing adjuvant ICI versus placebo/observation were selected. Study screening and selection, along with data extraction was performed in duplicate according to a predefined registered protocol. The Preferred Reporting Items for Systematic Reviews and Meta-analyses (PRISMA) reporting guideline was used. Quality assessment was performed using the Cochrane risk-of-bias (RoB 2) tool for randomized trials. The primary and secondary endpoints were DFS and serious adverse events, respectively. All outcomes were analyzed using random-effects meta-analysis owing to inter-study heterogeneity. Sensitivity and subgroup analyses were performed to identify potential sources of heterogeneity. A priori defined subgroups of interest included positive program death-ligand 1 (PD-L1) expression, previous use of neoadjuvant chemotherapy (NAC), primary tumor origin, pathologic lymph node status, and baseline Eastern Cooperative Oncology Group performance status. Pooled results across the 3 RCTs (2,220 patients) demonstrated significantly improved DFS for patients treated with ICI in the intention-to-treat cohorts (HR 0.76, 95% CI 0.65-0.90). There was considerable clinical and statistical heterogeneity (I