A 12-year-old female common mynah (Acridotheres tristis) was examined because of dyspnea and coelomic enlargement. Abdominal radiographs revealed hepatomegaly and a coelomic mass of unknown origin. Both brightness mode (2-dimensional; 2D) and 3-dimensional (3D) ultrasonography were performed to identify the origin of the mass. Ultrasonographic findings distinguished the mass from the liver and revealed high vascularization of the mass and a moderate increase in echogenicity of the liver. Three-dimensional ultrasonographic histogram analysis of the mass was performed. The mynah was treated with supportive care but died after 3 days. Histopathologic examination showed ovarian adenocarcinoma, with concurrent mild to moderate hepatopathy. In diagnostic imaging using ultrasound in birds, 3D ultrasonography allows perspective images of the internal organs to be obtained and is potentially superior to 2D ultrasonography in evaluating irregularly shaped objects.