Intraplacental leiomyomas are extremely rare and are generally incidental findings in term placentas. We present the first reported case of a placental leiomyoma associated with preterm intrauterine fetal demise, with histological findings providing the cause of adverse outcome. This was an intrauterine fetal demise detected at 26 weeks gestation with a placental finding of a 2.8-cm leiomyoma. Histological findings in the placenta and fetus were consistent with intrauterine fetal demise of weeks. The umbilical cord was markedly hypercoiled, with 6 twists per 10 cm. Features of maternal vascular malperfusion were evident in the placenta, including villous hypermaturity, an infarct adjacent to the leiomyoma, and retention of smooth muscle in spiral arterioles within the decidua overlying the leiomyoma. Implantation-site trophoblasts invaded into the leiomyoma and the overlying decidua. We hypothesize that incorporation of the leiomyoma into the placenta contributed to fetal demise due to disordered placental implantation, implying that these tumors may not be as benign and incidental as previously described. The finding of implantation-site changes in the leiomyoma may also suggest a potential cause for this rare tumor.