Chasmosaurus mariscalensis is a new species of ceratopsian dinosaur (Ornithischia; Ceratopsia) from the upper part of the Aguja Formation (late Campanian, Judithian) in Big Bend National Park, Brewster County, Texas. This species is distinguished from other species of Chasmosaurus by its relatively short and broad squamosal bearing six very large epoccipitals, maxilla without pronounced lateral shelf, premaxilla without posterodorsal extension, and very long supraorbital horncores in adults. A bone bed accumulation comprising disarticulated remains of 10-15 juvenile, subadult, and fully adult individuals forms the hypodigm of C. mariscalensis, and allows the first full description of the postcranial skeleton for the genus. Males and females are separated on the basis of brow horncore orientation. This is the most advanced species of Chasmosaurus and is morphologically intermediate with Pentaceratops in several characters.