The diaphragm is an uncommon site for solitary fibrous tumors of the pleura (SFTPs). Diaphragmatic SFTPs are usually found as pedunculated tumors with a clear margin with their surrounding structures. However, preoperative diagnosis of the tumor is difficult because their morphological features are not well detected on radiological images. We presented a case of diaphragmatic SFTP that was successfully diagnosed by ultrasonography (US). The US revealed a well-circumscribed mass with the feeding arteries leading from the diaphragm. Subsequently, this finding was confirmed histopathologically after resection. US can be less invasive and more sensitive than radiological examinations for diagnosing diaphragmatic SFTPs. To our knowledge, this is the first report of a preoperative diagnosis of diaphragmatic SFTP by US.