Complete androgen insensitivity syndrome (CAIS), also known as Morris syndrome, is a rare X-linked recessive disorder characterized by a 46XY karyotype and a female external phenotype. We present the case of a 32-year-old patient who presented to Unimed International Hospital in 2024 with primary amenorrhea, infertility, and chronic pelvic pain. Clinical examination, imaging, and laboratory investigations led to the diagnosis of CAIS. Laparoscopic surgery was performed to remove bilateral gonadal structures and a cystic mass on the left side. Histopathological analysis revealed testicular tissue and a serous cystadenoma originating from the left mesonephric remnant. Following gonadectomy, hormone replacement therapy was initiated, resulting in stabilization of hormone levels. This rare case highlights the possibility of mesonephric remnant-derived cystadenoma in CAIS and underscores the diagnostic value of cytogenetic and histological evaluations, especially in distinguishing between ovarian and testicular tissue when imaging findings are inconclusive.