We present a rare case of a 28-year-old man with a non-tender soft tissue enlargement on the dorsal side of the first web space of the right hand. Sonographic evaluation showed thickening of the dorsal thenar muscle (dorsal interosseous muscle) without a mass-like or cystic lesion, as compared with the left dorsal thenar muscle. On grasping position, this finding was more prominent and no significantly increased vascular flow was observed in this muscle. We were able to diagnose idiopathic hypertrophy of the first dorsal interosseous muscle solely by sonography. Ultrasonographic evaluation is a noninvasive and accessible tool for diagnosis of pseudotumors, such as idiopathic thenar muscle hypertrophy.