Fossils of Brontomerus were recovered from a quarry at the top of the Ruby Ranch Member of the Cedar Mountain Formation in eastern Utah, USA. The specimens were originally collected in 1994 and 1995 by a team of researchers from the Sam Noble museum of Norman, Oklahoma. It was named and described by Michael P. Taylor, Mathew J. Wedel and Richard L. Cifelli, an international team from the University College London, Western University of Health Sciences and the Sam Noble Oklahoma Museum of Natural History. The fossils were recovered from a locality known as the Hotel Mesa Quarry in easternmost Grand County, Utah. The site had been previously known to private collectors, who had already removed a considerable number of fossils, some of which may have been scientifically valuable. Exposed bones that remained were in various states of disrepair: some had even been broken and their pieces used to hold down a plastic tarpaulin. Due in part to this "pillaging" of the fossil site, the team was unable to recover more complete specimens.
Brontomerus is known from two fragmentary specimens differing in size, likely a juvenile and an adult. The researchers speculate that the adult may have been the mother of the juvenile. The type specimen, OMNH 66430, is represented by the left ilium of the smaller individual. Other recovered fossils include a crushed presacral centrum, several caudal vertebrae, a right-side dorsal rib, a large scapula, and two partial sternal plates. The adult specimen is thought to have weighed around six tons, and probably measured around 14 meters (46 ft) in length. The juvenile specimen had about a third of this length, and probably weighed around 200 kilograms and measured 4.5 meters (15 ft) in length.
Its assignment to a new species is based on several noteworthy autapomorphies, including an oddly-shaped hipbone which would have permitted the attachment of unusually massive leg muscles. This unique ilium would have given it the largest leg muscles of any sauropod dinosaur. The ilium is unusual in being very deep and having a front part that is much larger than the part behind the hip socket.