Ovarian adnexal masses are a common clinical challenge, often encountered in gynecological practice. These masses can range from benign cysts to malignant tumors, and their diagnosis requires careful evaluation due to the variety of potential etiologies. Clinically, adnexal masses present with nonspecific symptoms such as pelvic pain, bloating, or abnormal bleeding, though many are asymptomatic and discovered incidentally during imaging studies. The importance of differential diagnosis lies in distinguishing between benign and malignant conditions, as early detection of malignancy significantly improves prognosis. A thorough diagnostic approach, including imaging and histological assessment, is crucial. Among imaging techniques, three-dimensional ultrasound (3D-US) has emerged as a valuable tool in the differential diagnosis of ovarian adnexal masses. Three-dimensional ultrasound offers superior spatial resolution, allowing for better visualization of the mass’s size, shape, and internal structure. This can help differentiate between benign cystic formations, such as functional ovarian cysts, and more complex or suspicious masses that may require further investigation or surgical intervention. Additionally, 3D-US aids in assessing blood flow and vascularity, which can provide further clues to the nature of the mass. In this chapter, we shall review the role of 3D ultrasound in the differential diagnosis of adnexal masses.