It was named from a partial skeleton found in the Barremian-age Lower Cretaceous Lakota Formation of Custer County, South Dakota.

Although there was a push to synonymize it with Polacanthus in the late 1980s-early 1990s, Hoplitosaurus has been accepted as a valid albeit poorly known genus in more recent reviews.

[7] Ken Carpenter and James Kirkland noted that many of the resemblances between the two were more widely distributed among ankylosaurs than previously thought, or were based on damage to the bones, such as some femoral characters.

[7] Today, both are considered to be polacanthine or polacanthid ankylosaurs,[8] depending on classification preference (see for example [1]), or ankylosaurians of uncertain relationships.

[9] Blows (2001) reconsidered the armor in light of new data on polacanthine dinosaurs, and found it to fall into the following categories:[10]