A negative free-air gravity anomaly is associated with Nares Strait, the waterway that separates Ellesmere Island and Greenland. Two east–west gravity profiles that cross Ellesmere Island and Nares Strait were collected. A low with values in the range of −100 to −120 mGal (−1000 to −1200 μm/s2) was observed, and two-dimensional crustal models were created to identify the cause of the anomaly. The gravity anomaly cannot be attributed wholly to the bathymetry of the strait or to the sedimentary rocks underlying the strait. Crustal models that reproduce the anomaly have a M discontinuity that slopes under Nares Strait towards Ellesmere Island so that the crust beneath Ellesmere Island is thickened. The anomaly is similar to those associated with ancient and modern suture zones, regions of collided continental crust. Plate reconstructions suggest Nares Strait is a collisional boundary between the North American Plate (Ellesmere Island) and the Greenland Plate. The gravity anomaly supports this interpretation of Nares Strait.