Ianthasaurus hardestiorum, a basal edaphosaurid from the Upper Pennsylvanian of Garnett, Kansas, has been described on the basis of two incomplete, juvenile specimens and a series of disarticulated vertebral elements. New skeletal material of this poorly known species includes previously unknown cranial elements, increasing our knowledge of the anatomy and variation in this taxon. The complete ossification of the neural arches and the overall larger size of the vertebrae relative to those previously described indicate that they were part of an adult individual, and marginal tooth morphology resembles more closely that seen in the genus Edaphosaurus. Phylogenetic analysis of edaphosaurid synapsids confirms the hypothesis that Ianthasaurus is a sister-taxon of all other members of the clade. However, the incomplete fossil record of other putative edaphosaurids, such as Lupeosaurus and Glaucosaurus, makes full resolution of their phylogenetic interrelationships difficult to assess.