Purpose: To determine the prevalence, severity, and baseline associations of self-reported long-term unmet supportive care needs in a population-wide cohort of men with prostate cancer (PC), 15 years post-diagnosis. Methods: Participants were drawn from the New South Wales (NSW) Prostate Cancer Care and Outcomes Study. Eligible men were diagnosed with PC between 2000 and 2002, aged less than 70 years at diagnosis, and completed a 15-year follow-up survey. Demographic and clinical data were collected at baseline. The validated Cancer Survivors’ Unmet Needs (CaSUN) Survey was administered to assess unmet needs. Results: Of 578 eligible men, 351 completed CaSUN. Mean age was 75.8 (range 59–84) with a mean follow-up time of 15.2 years post-diagnosis. Over a third of men (37.4%) reported at least one unmet need at 15 years. Most frequently reported unmet needs pertained to the comprehensive cancer care (34.1%) domain. 87.2% of participants who reported problems with sexual function reported this need as moderate/severe. Higher diagnostic prostate-specific antigen (PSA) levels (20+ ng/mL) at diagnosis were associated with future unmet needs (PSA 20+: OR = 4.80, 95% CI [1.33–17.35]). Conclusion: Many PC survivors continue to report unmet needs 15 years post-diagnosis. There is a pressing need for clinicians to work together to coordinate PC care, and to proactively, regularly, and openly enquire about men’s sexual adjustment to PC. The needs of PC survivors could better be met with more coordinated approaches to multidisciplinary care and timely interventions and support for chronic sexual dysfunction.