Background: The purpose of this study was to evaluate the pathological and oncological significance of Gleason (G) 5 pattern in high-grade PCa after robotic-assisted radical prostatectomy (RARP). Materials and Methods: From a cohort of 1,046 men, 159 post-RARP patients by a single surgeon with pathological G8 (N = 79) and G9 (N = 80) met our inclusion criteria. G9 cancers were sub-stratified into G4+5 (N = 58) and G5+4 (N = 22). Clinical and pathological outcomes were evaluated with the t test or Mann-Whitney U test for continuous variables and the Pearson χ 2 test for categorical variables. The Kaplan-Meier method was used to estimate the biochemical recurrence-free survival (BCRFS), and survival curves were compared using the log-rank test. Multivariate analysis was performed with Cox regression analysis. Results: Baseline characteristics across all subgroups were similar except for number of positive cores on biopsy. There was a trend toward worse pathological and oncological outcomes in G9 cancers when compared with G8, although only tumor volume (TV), extracapsular extension (ECE) of tumor and lymph nodes involvement (LNI) achieved statistical significance. G4+5 PCa were statistically more likely to have ECE and a higher TV than G4+4 although the BCRFS were not significantly different. G5+4 cancers were associated with a significantly higher proportion of patients with LNI and had a statistically significant poorer BCRFS compared with G4+5 patients. Conclusions: Oncological and pathological outcomes of G8 were significantly better than G9 and merited distinction between them. G5+4 harbors a much poorer BCRFS compared with G4+5, and hence we suggest considerations for immediate adjuvant treatments.