Background: Platinum/S-1 (PS) and platinum/5-fluorouracil (PF) as first-line chemotherapies are extensively used for the treatment of advanced gastric or gastroesophageal junction cancer (AGC); however, there is no definite consensus on which regimen is best. In our meta-analysis, we compared PS with PF in terms of their efficacy and safety in AGC patients. Methods: PubMed, ScienceDirect, Web of Science, Scopus, Ovid MEDLINE, EMBASE, The Cochrane Library, Google Scholar, and CNKI were systematically searched for pertinent literature. We analyzed overall survival (OS), progression-free survival (PFS), objective response rate (ORR), disease control rate (DCR), and adverse effects (AEs) as major end points. Results: A total of 3,225 studies were identified, among which 6 randomized controlled trials, including 1,736 participants, were ultimately included in our analysis. Our results showed that PS and PF were comparable in terms of OS (p = 0.33, 95% confidence interval [CI]: 0.84-1.06), PFS (p = 0.63, 95% CI: 0.87-1.09), ORR (p = 0.38, 95% CI: 0.91-1.28), DCR (p = 0.41, 95% CI: 0.86-1.43), total AEs (p = 0.41, 95% CI: 0.98-1.01), and grade ≥3 AEs (p = 0.58, 95% CI: 0.82-1.41). However, those who received PF had a shorter time to failure (TTF) (p = 0.01, 95% CI: 0.77-0.97), and a significantly higher rate and more severe cases of stomatitis, nausea, and hypokalemia were reported in the PF group. Conclusions: PF and PS show similar antitumor efficacy (OS, PFS, ORR, and DCR), but patients receiving PS exhibit longer TTF and fewer AEs (stomatitis, nausea, and hypokalemia) than those receiving PF.