Ovarian mucinous borderline tumors are traditionally divided into 2 morphologic groups: endocervical type, also known as seromucinous, and intestinal type. We present a case of stage 4B seromucinous ovarian borderline tumor with endometriosis and review the literature. At the time of writing, this is believed to be the first case of a seromucinous borderline tumor reported at such an advanced stage.