Pseudomyxoma peritonei (PMP) is a rare condition predominantly originating from appendiceal neoplasms, making true primary ovarian PMP an exceptional and widely debated clinical entity. Establishing a primary Müllerian origin requires exhaustive histological and immunohistochemical evaluation to exclude an occult appendiceal tumor. A highly unusual clinical scenario involves a 26-year-old nulliparous woman presenting with a large borderline mucinous ovarian tumor paradoxically associated with high-grade peritoneal implants. Comprehensive pathological workup (CK7+, PAX8+, CK20−, CDX2−) and completely normal appendiceal serial sections definitively confirmed the primary ovarian origin. Standard-of-care management relies on complete cytoreductive surgery (CRS) combined with hyperthermic intraperitoneal chemotherapy (HIPEC). Successful CC0 cytoreduction and HIPEC were achieved, integrated with proactive ovarian cortex cryopreservation. This unusual presentation highlights the striking biological discordance that can exist between a primary tumor and its peritoneal dissemination, emphasizing the critical role of specialized multidisciplinary management, rigorous pathology, and early oncofertility integration for young patients.