We report the results of interposition tendon grafts using the ipsilateral palmaris longus tendon in 12 patients with closed flexor digitorum profundus tendon ruptures in zone III or IV of 14 digits between June 2006 and October 2015. Before surgery, 2 patients were diagnosed with closed tendon ruptures that occurred after nonunion of hamate hook fractures. The other 10 patients were diagnosed with spontaneous tendon ruptures of unknown cause. In 2 of the 10 patients with spontaneous tendon rupture, the cause of the rupture was not found. In the other 8 patients, there was rough surface with deficient overlying soft tissue on the radial side of the hamate hook. In all cases, the ruptured flexor digitorum profundus was reconstructed by applying overtension on the tendon graft, causing greater flexion than for the other normal digits. Hamate hook excision was also performed on 10 subjects with abnormalities. Postoperatively, the patients were followed for an average of 22.5 months (range, 12–64 months). At the final follow-up, the mean Disabilities of the Arm, Shoulder, and Hand questionnaire score was 5.7 (range, 3.3–8.3). There were excellent results in all 14 digits according to Strickland and Glogovac criteria. The mean total active motion was 167 degrees (range, 160–180 degrees). There were no surgical complications, including infection, adhesions, or tendon rerupture. There were excellent clinical results with the interposition tendon graft using palmaris longus for closed tendon rupture in zone III or IV of the hand. Applying overtension to the grafted tendon appears to be beneficial.