Background: PD-1/PD-L1 inhibitor-based monotherapies and combination therapies are being investigated to challenge microsatellite stable (MSS) mCRC. This NMA aimed to assess the efficacy of anti-PD-1/PD-L1-based therapies against active comparators (regorafenib, trifiuridine+tipiracil, and fruquintinib [R/T/F]) and negative controls (placebo and best supportive care [P/BSC]) in patients with refractory MSI-unselected mCRC, which are predominantly MSS. Methods: We searched PubMed, EMBASE, the Cochrane Central Register of Controlled Trials, and major oncology websites for relevant RCTs comparing the above therapies with each other up to Feb 15, 2019. We used the frequentist NMA to evaluate hazard ratios (HRs) for pairwise treatment comparisons in terms of overall survival (OS). Results: Seven RCTs were included, totaling nine comparisons between five therapies (durvalumab+tremelimumab [anti-PD-L1+Tre] vs. P/BSC [n = 1], atezolizumab+cobimetinib [anti-PD-L1+Cobi] vs. R/T/F [n = 1], atezolizumab [anti-PD-L1-only] vs. R/T/F [n = 1], anti-PD-L1+Cobi vs. anti-PD-L1-only [n = 1], and R/T/F vs. P/BSC [n = 5]). No significant heterogeneity was detected in the NMA (P = 0.340 in Q test; I²= 11.6%). Compared to P/BSC, OS was significantly improved with anti-PD-L1+Tre (fixed-effects model HR, 0.72 [95% confidence interval, 0.54-0.96]; P = 0.014) and anti-PD-L1+Cobi (HR, 0.70 [0.50-0.98]; P = 0.018), but not with anti-PD-L1-only (HR, 0.83 [0.57-1.21]; P = 0.172). OS with anti-PD-L1+Tre and anti-PD-L1+Cobi did not significantly differ from that with R/T/F (P = 0.565 and P = 0.500, respectively). Conclusions: In refractory MSI-unselected mCRC, anti-PD-L1 monotherapy was ineffective but anti-PD-L1+Tre and anti-PD-L1+Cobi therapies exhibited efficacies comparable to that of R/T/F.