A complete septate uterus, resulting from the abnormal fusion of the Müllerian ducts, is a rare uterine anomaly in which the uterine cavity is divided by a septum, resulting in two separate uterine cavities. In this case, we encountered a patient with a complete septate uterus complicated by a dichorionic diamniotic twin pregnancy, with each fetus developing in separate uterine cavities. During the course of the pregnancy, the placenta of the trailing twin (Twin B) was attached to the septum. As the pregnancy progressed, the septum became progressively thinner, and Twin B exhibited growth restriction.