New zircon and monazite U-Pb data, tectonic mapping, and petrologic studies in key units of the Acatlán Complex show a previously undocumented phase of continental collision orogeny of Late Ordovician–Early Silurian age in southern Mexico. The event involved the partial eclogitization of oceanic lithosphere and continental crust, which traveled westward more than 200 km over siliciclastic metasedimentary rocks of the trench-forearc of an opposing continental margin. The overriding eastern margin was the Oaxaquia microplate attached to Gondwana, and the western overridden margin is considered to have been the eastern margin of Laurentia. This event, which we name the Acatecan orogeny, was roughly synchronous with the possible closure of Iapetus along the Appalachian margin, which involved, according to current models, either the docking of peri-Gondwanan terranes such as Avalonia and Carolina or the direct collision between Gondwana and Laurentia. The permanence of Oaxaquia in northwestern Gondwana until the end of the Silurian, as suggested by Tremadocian to Silurian marine faunas in the cover of Oaxaquia, is more consistent with the direct collision of Gondwana and Laurentia at the end of the Ordovician, forming the Acatlán Complex between.