Five species of Glyptothorax are identified from the Mae Klong River basin in western Thailand, only one of which, G. buchanani, was previously reported from the basin; others are G. lampris, G. longinema, G. platypogonides, and G. schmidti. The morphological differences delineating species of Glyptothorax in the Mae Klong were visualized using principal component analysis of data taken from 105 specimens, and the latitudinal range and number of river basins over which all occur were found to be greater than previously recognized. Glyptothorax platypogonides, recently thought to range only as far north as peninsular Thailand, occurs in the Mae Klong and Chao Phraya basins in western and northern Thailand. Glyptothorax schmidti, thought to occur only as far north as the Tanintharyi River, Myanmar, was found to be much more wide-ranging, with the inclusion of populations in the Mae Klong, Chao Phraya, and Mekong River basins. Glyptothorax longinema was found to occur outside of China in lower portions of the Salween River basin and in the Mae Klong basin. The presence of G. lampris in the Chao Phraya is questioned and its southern distributional limit was extended to the Tapi River basin. Phylogenetic reconstruction of COI sequence data for 127 individuals from across Southeast Asia revealed strong BI and ML support for recognition of 14 species in Thailand and suggested deep-rooted clades that are discussed as corresponding broadly to body coloration. Glyptothorax callopterus was found to be genetically distinct from G. fuscus and is resurrected from synonymy. A key to all species of Glyptothorax in Thailand is provided.