Granular cell tumor (GCT) represents a less frequently seen tumor originating from Schwann cells. Although GCT develops in various locations in the human body, GCT of the mediastinum is extremely uncommon. A case of mediastinal GCT diagnosed by aspiration using a fine needle assisted by endoscopic ultrasound (EUS‐FNA) via a modified technique based on wet suction was reported. An asymptomatic 28‐year‐old man was referred for assessment of a mass in the mediastinum that was found incidentally via chest computed tomography (CT) at health screening. EUS demonstrated a hypoechoic lesion with a distinct boundary, which was derived from the upper posterior mediastinum and partly located close to the posterior wall of the esophagus. Therefore, EUS‐FNA with a modified wet suction technique was performed to harvest adequate specimens for the diagnosis of GCT. Minimally invasive tumor removal was performed, and histological examination of the specimen harvested surgically verified GCT, consistent with histological findings of the specimen obtained by EUS‐FNA. The case highlights that a good accuracy of histological diagnosis and specimen quality are achieved for the modified wet‐suction technique in EUS‐FNA, and a preoperative diagnosis of mediastinal GCT can be made with certainty.