Glyptosaurinae is an extinct group of anguid lizards commonly found in the Cretaceous and Paleogene fossil record of North America and Eurasia. Glyptosaurines are well documented up until the middle Oligocene, but the temporal extent of the group was never constrained precisely. Here, I document the youngest known record of glyptosaurine lizards. I describe 10 fossils of the glyptosaurine Peltosaurus granulosus from the Monroe Creek and upper Sharps formations of Sharps Corner, South Dakota. The youngest known occurrence of Peltosaurus and Glyptosaurinae is between 27.4 and 26.4 Ma, based on a correlation of the magnetozone of the Monroe Creek Formation with chron C9n. Those dates are in the age range of a hypothesized middle–late Oligocene warming event. I review relevant morphological characteristics previously described for Peltosaurus and provide images of the holotypes of Peltosaurus abbotti and Peltosaurus granulosus, as well as two relatively complete specimens of Peltosaurus granulosus. I also establish an apomorphy of Peltosaurus on the frontal bone. Additionally, I correct an error in the literature pertaining to the original description of Peltosaurus.