Complete androgen insensitivity syndrome is a rare 46,XY disorder of sex development caused by mutations in the androgen receptor gene, resulting in androgen resistance despite a normal male karyotype. Individuals with complete androgen insensitivity syndrome typically present with female external genitalia, primary amenorrhea, and a heightened risk of gonadal germ cell tumors. Herein, we report the case of a 30-year-old woman who was diagnosed with complete androgen insensitivity syndrome at 18 years of age during evaluation for primary amenorrhea. Chromosomal analysis revealed a 46,XY karyotype, and imaging confirmed the absence of a uterus and ovaries. Despite medical advice for prophylactic gonadectomy, the patient delayed surgery and later presented with a palpable abdominal mass. Genetic analysis identified a novel hemizygous germline missense mutation, c.1768G>T (p.G590W), in exon 2 of the androgen receptor gene, which was classified as potentially pathogenic based on the American College of Medical Genetics and Genomics criteria. The patient underwent tumor resection followed by four cycles of bleomycin, etoposide, cisplatin chemotherapy regimen for advanced seminoma and has remained disease-free during follow-up. This case underscores the importance of genetic analysis, early prophylactic gonadectomy, and multidisciplinary care in managing complete androgen insensitivity syndrome to mitigate tumor risk and optimize outcomes.