Anopheles baileyi species D of the Baileyi Complex, subgenus Anopheles (Diptera: Culicidae) in Thailand is diagnosed and formally named An. inthanonensis Somboon & Harbach, n. sp. Morphological characters of the adults, and the pupal and larval stages with their chaetotaxy, are provided and compared with other species of the complex. Phylogenetic analysis of COI sequences revealed that An. inthanonensis appears to be more closely related to An. monticola in Bhutan and China than it is to other members of the Baileyi Complex.