Complete androgen insensitivity syndrome (CAIS) is caused by mutations in the androgen receptor gene. Patients with this syndrome have a 46,XY karyotype, male gonads, and normal female external genitalia. While the pre-pubertal risk of developing gonadal tumors is low in these patients, it increases with age. Most gonadal tumors arise from germ cells; stromal cell tumors are uncommon. Herein, we report a CAIS patient with a feminizing Sertoli cell tumor. The patient presented at 8 yr of age with breast enlargement and growth acceleration, concomitant with elevated serum estradiol levels and suppressed serum gonadotropin levels; these findings were inconsistent with CAIS. The patient underwent gonadectomy at 10 yr of age, and histology demonstrated presence of a non-malignant Sertoli cell tumor in the right gonad. We conclude that this is the first reported case of CAIS with accelerated onset of puberty resulting from a Sertoli cell tumor.