The North American Cordillera has been shaped by a long history of accretion of arcs and other buoyant crustal fragments to the western margin of the North American plate since early Mesozoic time. The southernmost accreted terrane is the Guerrero terrane of southwestern Mexico, a latest Jurassic−Cretaceous volcanic arc built on a Triassic accretionary prism. Interpretations of the origin of the Guerrero terrane vary: Some authors consider it a far-traveled, exotic intra-oceanic island arc, while others view it as the (par)autochthonous, extended North American continental margin. We present new paleomagnetic and U-Pb zircon data from Lower Cretaceous sedimentary rocks of the Guerrero terrane. These data show that the Guerrero terrane has a latitudinal plate-motion history equal to that of the North America plate, both before and after accretion. This confirms paleogeographic models in which the Guerrero arc successions formed on North American crust that rifted away from the Mexican mainland by approximately east-west opening of a back-arc basin above an eastward-dipping subduction zone. Additionally, it renders alternative paleogeographical models in which the Guerrero terrane is considered to be exotic to the North American continent unlikely. The phase of back-arc spreading resulted in the short-lived existence of an additional “Guerrero” tectonic plate between the North American and Farallon plates and, upon closure of the back-arc basin, the growth of the North American continent.