Ovarian seromucinous carcinoma (SMC) is an uncommon neoplasia and is composed predominantly of serous and endocervical-type mucinous epithelium. Due to its low frequency and difficult diagnosis, the natural history, characteristic imaging findings, and pathological features of SMC have not been adequately described in the literature thus far. We herein report three cases of ovarian SMC along with magnetic resonance imaging (MRI) findings. The diagnosis of SMC was made after staging laparotomy in all cases, and systemic chemotherapy was performed in two cases. No recurrence was observed in any of the cases. The MRI findings in SMC were so varied that characteristic imaging features useful for diagnosis were not found. In two cases, MRI suggested endometriotic cysts, and endometriosis and seromucinous borderline tumors (SMBTs) were detected concurrently in all cases by histological examination. Thus, it was suggested that SMC develops in multiple stages via endometriosis and SMBT. The cooccurrence of endometriosis and SMBT could also make the diagnosis of SMC more convincing.