Background There is limited and controversial evidence on the prognosis of partial nephrectomy (PN) versus radical nephrectomy (RN) in patients with T3aN0/xM0 renal cell carcinoma (RCC) upstaged from clinical T1 RCC. In this study, we aimed to assess the prognosis difference following PN versus RN in patients with <= 7 cm T3aN0/xM0 RCC. Methods From the Surveillance, Epidemiology, and End Results database, a total of 3196 patients receiving treatment of PN/RN for <= 7 cm T3aN0/xM0 RCC with only extrarenal fat extension in 2010-2017 were identified. An inverse probability of treatment weighting (IPTW)-adjusted cause-specific Cox model with hazard ratio (HR) and 95% confidence interval (CI) was used for overall survival (OS) and cancer-specific survival (CSS) analyses. Sensitivity analysis was based on the propensity score matching of PN and RN groups and from the dataset of 2010-2013. Results A total of 872 patients underwent PN, compared with 2324 undergoing RN. After IPTW adjustment, there was no significant difference in preoperative baseline characteristics between the PN and RN cohorts. Patients who underwent RN had worse OS (HRIPTW-adjusted, 1.46; 95% CI, 1.16-1.84; p = 0.001) and comparable CSS (HRIPTW-adjusted, 1.03; 95% CI, 0.64-1.66; p = 0.890) than those receiving PN in all cohorts and subgroups with T3a RCC of <= 4 cm and perinephric fat extension. Further, in patients with 4-7 cm T3a RCC with perinephric-fat invasion and all sizes of T3a RCC with sinus/perisinus fat extension, PN led to comparable OS and CSS. Sensitivity analyses validated these results. Conclusion PN provides comparable CSS and OS or even better OS than RN for patients with RCC <= 7 cm T3aN0/xM0. Although our study has some limitations, our results indicated that PN might oncologically safe for clinical T1 RCC, even confirmed a pathologically T3a upstaging post-PN.