Frog virus 3 (FV3) and related ranaviruses are emerging infectious disease threats to ectothermic vertebrate species globally. Although the impact of these viruses on amphibian health is relatively well studied, less is understood about their effects on reptile health. We report two cases of FV3 infection, 11 mo apart, in three-toed box turtles (Terrapene mexicana triunguis) from a wildlife rehabilitation center. Case 1 had upper respiratory signs upon intake but had no clinical signs at the time of euthanasia 1 mo later. Case 2 presented for vehicular trauma, had ulcerative pharyngitis and glossitis, and died overnight. In case 1, we detected FV3 nucleic acid with qPCR in oral swabs, kidney, liver, spleen, and tongue. In case 2, we detected FV3 in an oral swab, an oral plaque, heart, kidney, lung, liver, spleen, and tongue. We also detected FV3 nucleic acid with in situ hybridization for case 2. For both cases, FV3 was isolated in cell culture and identified with DNA sequencing. Histopathologic examination of postmortem tissue from case 1 was unremarkable, whereas acute hemorrhagic pneumonia and splenic necrosis were noted in case 2. The difference in clinical signs between the two cases may have been due to differences in the temporal course of FV3 disease at the time of necropsy. Failure to detect this infection previously in Missouri reptiles may be due to lack of surveillance, although cases may also represent a novel spillover to box turtles in Missouri. Our findings reiterate previous suggestions that the range of FV3 infection may be greater than previously documented and that infection may occur in host species yet to be tested.