No head-to-head postmarket surveillance study has compared the differences in adverse events (AEs) between two combination therapies, axitinib (AXI) + pembrolizumab (PEMBRO) and lenvatinib (LEN) + PEMBRO, against metastatic renal cell carcinoma. This study aims to highlight the comprehensive differences in AEs between these two therapies based on the real-world big data from the Food and Drug Administration Adverse Event Reporting System (FAERS) database. In total, 28 937 records were extracted from the FAERS database, and 139 AEs grouped into the System Organ Class according to the Medical Dictionary for Regulatory Activities were analysed. Logistic regression analyses were performed, and the reporting odds ratio with a 95% confidence interval was determined. We found that the incidences of cardiac and hepatobiliary disorders for AXI + PEMBRO, and blood and lymphatic system, metabolism and nutrition, and vascular disorders for LEN + PEMBRO, all of which were associated with serious AEs, were higher than those for LEN + PEMBRO and AXI + PEMBRO, respectively. The differences in the AEs between AXI + PEMBRO and LEN + PEMBRO were not derived merely from those between AXI and LEN monotherapies. Furthermore, remarkable AE potentiation was observed for AXI + PEMBRO. As FAERS is a spontaneous reporting system comprising partially limited information, analysing more detailed relationships between AEs and patient or treatment characteristics was challenging in this study. The present study is the first to show the overall real-world postmarketing differences in AEs between AXI + PEMBRO and LEN + PEMBRO. Our novel findings will substantially improve clinical practice; we recommend comparing patients' conditions associated with the above AEs when selecting between these two therapies. PATIENT SUMMARY: Herein, we highlight the differences in adverse events (AEs) between axitinib + pembrolizumab and lenvatinib + pembrolizumab therapies using data from the real-world Food and Drug Administration Adverse Event Reporting System database aimed at patients with metastatic renal cell carcinoma. We identified AEs that needed attention in each combination. We recommend the differences in AEs to be considered when selecting these two therapies.