Prostate cancer (PC) is the most frequently occurring cancer in men, characterized by the abnormal proliferation of cells within the prostate gland. This study explores the role of RNA binding motif protein 15B (RBM15B) in PC. RBM15B expression levels in PC patients were predicted using the Starbase database. The expression of RBM15B and proliferating cell nuclear antigen (PCNA) expression in PC cells was detected. Following RBM15B knockdown, cell proliferation assays were conducted. N6-methyladenosine (m6A) levels in PC cells were quantified, and RNA immunoprecipitation was performed to analyze the binding of m6A and YTH N-methyladenosine RNA binding protein 1 (YTHDF1) on PCNA mRNA. The stability of PCNA mRNA was assessed after treatment with actinomycin D. An in vivo nude mouse xenograft model was created to validate the role of RBM15B. The findings revealed the upregulation of RBM15B in PC. RBM15B knockdown resulted in decreased proliferation, colony formation, and EdU-positive cells. Mechanical analysis showed that RBM15B facilitated m6A modification of PCNA mRNA, leading to increasing m6A methylation. YTHDF1 bound to these m6A sites on PCNA mRNA, thus stabilizing it. Furthermore, PCNA overexpression mitigated the effects of RBM15B knockdown on PC cell proliferation. In conclusion, RBM15B promotes PC cell proliferation by enhancing the stability of PCNA mRNA through YTHDF1-mediated m6A modification.