A case of ovarian sex cord tumor with annular tubules (SCTAT) without Peutz‐Jeghers syndrome is presented in which portions of the neoplasm showed endometrioid differentiation. In addition, histologic examination of endometrium obtained from curettings and hysterectomy specimen revealed atrophic glands and a decidual type stroma, indicating a possible gestagenic effect of the ovarian tumor. The latter has so far been only briefly alluded to in the literature, whereas to the best of the authors' knowledge, endometrioid differentiation has not yet been reported to occur in SCTAT.