Purpose: First web space widening is crucial in the hand function. The skin on the dorsal side of the forearm can provide a thin and pliable skin suitable for first web space reconstruction. Although previous reports have described the use of the posterior interosseous artery (PIA) flap as a reverse-flow flap for treatment of first web space contracture, only a few have addressed its use as a free flap for this purpose. The caliber of the concomitant veins accompanying the PIA is usually small, which may give rise to a problem in the treatment. Methods: Seven patients with first web space contracture were treated with a free PIA flap and the details of the venous anastomosis method were elucidated. Results: Six of seven flaps survived. In a post-burn case, a flap was lost by late thrombosis. The PIA is anastomosed endto-end to the dorsal branch of the radial artery. There are two choices for the recipient venous pedicle: concomitant veins of radial artery and a tributary of the cephalic vein. In our cases, there were four types of venous anastomosis. An average postoperative increase of the thumb radial abduction was 36° and that of the palmar abduction was 35°. Conclusion: Since the caliber of the concomitant veins accompanying the PIA is small, a careful scheme for venous anastomosis is essential in the treatment of first web space contracture using the free PIA flap. Key Words: Free posterior interosseous artery flap, First web space contracture, Venous anastomosis Arch Hand Microsurg 2019;24(4):408-415.