The Trans Vietnam Orogenic Belt (TVOB) of Southeast Asia is thought to have formed by a continent-continent collision between the South China and Indochina cratons in Permian-Triassic times. Here we focus on the nature and origin of metamorphosed mafic rocks that are widely distributed throughout the TVOB, including rocks from the Cangshan Mountains (northern TVOB), the Red River Shear Zone and Song Ma Suture Zone (central TVOB), and the Kontum Massif (southern TVOB). Amphibolite facies metamorphosed mafic rocks are widely distributed throughout all of these areas, while granulite facies and eclogite facies metamorphosed mafic rocks occur only in the Kontum Massif and Song Ma Suture Zone, respectively. Major and trace element compositions of these TVOB metamorphosed mafic rocks indicate an overall tholeiitic affinity, but suggest a wide array of tectonic settings for the precursor mafic magmas, including volcanic-arc basalt (VAB), mid-ocean ridge basalt (MORB), and within-plate basalt (WPB). In addition, Fe-rich gabbro-derived mafic metamorphic rocks occur in the southern part of the TVOB (in the Kontum Massif). On the basis of the distributions of each type of mafic metamorphic rocks and their metamorphic grades, there were arc and oceanic crust between the South China Craton and the Indochina Craton.