Ovarian mucinous borderline tumor of müllerian type (MMBT) and mixed epithelial borderline tumor of müllerian type (MEBT) are uncommon subtypes of ovarian surface epithelial tumors. Both are often associated with endometriosis, but their histogenesis is still debated. We have noticed occasional foci of subepithelial cuboidal cells resembling uterine cervical reserve cells (RCs) in MMBTs/MEBTs, which have not been documented in the literature to the best of our knowledge. This study was carried out to identify the presence of RC-like cells (RCLCs) in MMBTs/MEBTs and their immunohistochemical features in comparison to those of cervical RCs. We analyzed 10 consecutive cases of RC-like MMBTs/MEBTs, 6 of which were associated with endometriosis. Immunohistochemistry was performed for p63, cytokeratin 34BE12, cytokeratin 17 (CK17), and low-molecular cytokeratin CAM5.2. In 9 of 10 cases, RCLCs were appreciated in hematoxylin-eosin stain, although their amount in the tumor varied from case to case. Immunohistochemically, RCLCs were positive for p63 in 9 cases. They were positive for both 34BE12 and CK17 and were very weakly positive or negative for CAM5.2 in 8 cases. This immunohistochemical profile is similar to that seen in the cervical RCs. Reserve cell-like cells were also found in the foci of endometriosis coexisting with MMBTs/MEBTs in 1 of 5 cases examined. We draw attention to the existence of the RCLCs in MMBTs/MEBTs and in endometriosis. Their similarity to the cervical RCs may indicate their potential role as precursor cell that may subsequently differentiate into different müllerian cell types, thus merit further study.