We describe a new species of groundsnake of the genus Stegonotus (Colubridae) from the Purari River basin in Gulf Province, Papua New Guinea. The new species can be most readily distinguished from all other New Guinean Stegonotus by its unique dorsal colour pattern which consists of a dark head and creamy-white anterior one third to two thirds of the body, grading into increasingly dense dark pigmentation on the posterior of the body and tail. It is most similar to S. iridis from the Raja Ampat Archipelago off western New Guinea, but that species has a different pattern of pigmentation dorsally, has a lower ventral scale count (198-211 vs. 229-239), and exhibits a different temporal scale arrangement. The description of S. aplini sp. nov. brings to fourteen the number of Stegonotus species described from New Guinea. A dichotomous key to described species in the New Guinea region is provided.