Adnexal masses detected during pregnancy require a prompt and accurate diagnosis to ensure fetal safety and good oncological outcomes. Computed tomography is the most common and useful diagnostic imaging modality for diagnosing adnexal masses; however, it is contraindicated in pregnant women because of the teratogenic effect of radiation on the fetus. Therefore, ultrasonography (US) is commonly used as the main alternative for the differential diagnosis of adnexal masses during pregnancy. Additionally, magnetic resonance imaging (MRI) can assist in the diagnosis when US findings are inconclusive. As each disease has characteristic US and MRI findings, understanding these features is important for the initial diagnosis and subsequent treatment. Thus, we thoroughly reviewed the literature and summarized the key findings of US and MRI to apply these in real-world clinical practice for various adnexal masses detected during pregnancy.