North America hosted a diverse assemblage of horned dinosaurs from the late Campanian until the end of the Cretaceous, but comparatively little is known about earlier horned dinosaurs. This paper reports on previously undescribed ceratopsian remains from the middle Campanian beds of the Judith River Formation of Montana, which represent the oldest known chasmosaurine. The Judith River chasmosaur shows a combination of characters not seen in any previously described ceratopsid. The parietal has a broad median bar, a rounded caudal margin, and highly reduced epiparietals. Episquamosals are enlarged anteriorly but decrease in size posteriorly, and imbricate as in centrosaurines. The postorbital horns are moderately elongate, inclined anterolaterally, and have an unusual teardrop-shaped cross section. The unique combination of characters seen in the Judith River chasmosaurine precludes referral to any previously known genus, and it is therefore described as a new genus and species, Judiceratops tigris. The addition of Judiceratops to the dinosaurian fauna of North America underscores the diversity of horned dinosaurs in the Late Cretaceous, which results from a combination of high diversity within faunas, a high degree of endemism, and rapid faunal turnover.