Collision tumor is defined as the coexistence of two adjacent, but histologically distinct tumors without histological admixture in the same tissue or organ. Collision tumors involving ovaries are extremely rare. The coexistence of a mucinous cystadenoma with a dermoid cyst is infrequently reported. However, the most common histological combination of collision tumor in the ovary is the coexistence of teratoma with mucinous tumors. If a dermoid cyst accompanies a multiseptated cyst and if the multiseptalcyst contains fatty foci, these two components may be associated. Recognizing the potential for the coexistence of these two neoplasms in the same ovary is essential to be able to make a correct diagnosis.