We report a new tree frog of the genus Gracixalus from western Thailand and describe it as a new species Gracixalus seesom based on results of morphological and molecular analyses. The new species is a small-sized Gracixalus (male snout-vent length ca. 22 mm) and is morphologically similar to G. gracilipes, but is easily distinguished from it by its dorsal tan color in life, absence of white spot on lower lip, and black markings on its foot webbing. The new species also clearly differs from all the other members of the genus by the combination of small body size, triangular snout, and light yellowish brown dorsum without distinct tuberculations. Problems of phylogeny and taxonomy of the genus Gracixalus are briefly discussed.