Widespread and excessive use of smartphones is very common, and its overuse is associated with several health-related conditions. A 58-year-old man presented with a spontaneous swan-neck deformity of the third digit. On his physical examination, full passive range of motion of the finger was achieved, while active range of motion was limited due to pain. Laboratory test results were normal. Ultrasonographic imaging demonstrated digital extensor tenosynovitis of the second, third, fourth, and fifth digits. Plain radiographs of bilateral hands were normal. Magnetic resonance imaging of the affected hand revealed no mass lesion. He attended to the hand rehabilitation program. At the end of the program, his pain decreased, and he was easily able to do the finger range of motion exercises. In conclusion, smartphone addiction has an adverse inf luence on hand function and pinch strength. Youngsters should be aware of the harmful effects which may result from smartphone overuse.