Germ cell tumors are the second most common group of ovarian neoplasms. Mature cystic teratoma is the most common and benign germ cell tumor. US findings of teratoma are various and sometimes mimic other adnexal masses. Computed tomography (CT) and magnetic resonance imaging (MRI) diagnosis of ovarian teratoma depends on the detection of fatty component. Imaging findings of ovarian mature cystic teratomas may be complicated by torsion, rupture, malignant transformation, or coexisting tumors. Immature teratoma affects younger patients and differs from mature cystic teratoma in imaging findings including larger size, enhancing solid components, scattered calcifications, and fat. Struma ovarii is the most common monodermal teratoma and may show characteristic CT and MRI findings due to thyroid component. Malignant germ cell tumors are the most common ovarian cancers in the first two decades of life. Dysgerminoma is the most common malignant germ cell tumor, and a multilobulated solid mass with fibrovascular septa is the characteristic finding. Endodermal sinus tumor is a high malignant germ cell tumor with hemorrhage and necrosis on imaging. Imaging findings of malignant mixed germ cell tumor may represent their constituents.