Although the presence of tertiary Gleason pattern 5 is reportedly related to unfavorable prostate cancer characteristics, few data are available regarding the effects of tertiary Gleason pattern 5 on the new ISUP (International Society of Urological Pathology) grading system in radical prostatectomy patients. In this study, we evaluated the effect of tertiary Gleason pattern 5 on biochemical recurrence following radical prostatectomy in patients with prostate cancer. We retrospectively evaluated 1000 patients with prostate cancer who underwent radical prostatectomy. The ISUP Grades were as follows: Grade 1, 16.3%; Grade 2, 48.1% (with tertiary Gleason pattern 5, 8.0% and without tertiary Gleason pattern 5, 40.1%); Grade 3, 21.9% (with tertiary Gleason pattern 5, 9.1% and without tertiary Gleason pattern 5, 12.8%); Grade 4, 3.9%; and Grade 5, 9.8%. Biochemical recurrence-free survival differed significantly among the five groups (Grade 1-5) (P < 0.0001). Grade 2 with tertiary Gleason pattern 5 had a significantly worse prognosis than Grade 2 without tertiary Gleason pattern 5. Similarly, Grade 3 with tertiary Gleason pattern 5 demonstrated a significantly worse prognosis than Grade 3 without tertiary Gleason pattern 5. When Grade 2 and Grade 3 were divided with or without tertiary Gleason pattern 5, the survival curves differed significantly among Grade 1, Grade 2 without tertiary Gleason pattern 5, Grade 2 with tertiary Gleason pattern 5, Grade 3 without tertiary Gleason pattern 5, Grade 3 with tertiary Gleason pattern 5, and Grade 4 (P < 0.0001) (hazard ratios: 1, 1.14, 1.86, 2.23, 3.53, and 6.30). In a multivariate analysis, integrating tertiary Gleason pattern 5 into the ISUP Grade, PSA, and surgical margin status significantly predicted biochemical recurrence (P < 0.0001). Integrating tertiary Gleason pattern 5 into the ISUP grading system will improve the accuracy of patient outcome prediction following radical prostatectomy in patients with prostate cancer.