The proposed international staging collaboration for cancer of the prostate (STAR-CAP) clinical prognostic system for prostate cancer predicts cancer-specific mortality (CSM) for patients for whom active treatment, such as radical prostatectomy (RP), is planned. Until now, no validation of STAR-CAP has been performed. We retrospectively analyzed data from our institutional database for 19 552 patients treated with RP between 1992 and 2015. We applied the STAR-CAP point assignment criteria to calculate total individual scores and then classified patients according to the STAR-CAP stage groups ranging from IA (lowest risk) to IIIC (highest risk). We evaluated biochemical recurrence (BCR)-free survival, metastasis-free survival (MFS), and cancer-specific survival (CSS) stratified by STAR-CAP stage groups over 10 yr, calculated the area under the receiver operating characteristics curve (AUC), and performed decision curve analyses to assess the ability of STAR-CAP to predict these outcomes after fitting the data from our single-institution data set. STAR-CAP performed well in stratifying individual survival outcomes for BCR-free survival, MFS, and CSS for each stage group in Kaplan-Meier analyses (p < 0.001 between groups). The AUC for prediction of BCR, metastasis, and CSM at 10 yr was 0.73, 0.84, and 0.75, respectively. Our findings validate the performance of STAR-CAP for European patients treated with RP. PATIENT SUMMARY: We validated the STAR-CAP system for predicting cancer outcomes after removal of the prostate. Our results show that the system performs well and could help in counseling patients with prostate cancer.