Thescelosaurus was named by Gilmore (1913). It is the type genus of Thescelosaurinae, Thescelosauridae.

It was assigned to Scelidosaurinae by Huene (1927); to Camptosaurinae by Nopcsa (1928) and Huene (1934); to Camptosauridae by Gilmore (1913) and Huene (1956); to Ornithopoda by Ostrom (1965); to Iguanodontidae by Matthew (1915), Galton (1974), Galton (1974) and Thulborn (1975); to Hypsilophodontia by Sereno (1986) and Norman (1989); to Iguanodontia by Buchholz (2002); to Euornithopoda by Norman et al. (2004) and Galton (2009); to Neornithischia by Boyd et al. (2009); to Ornithopoda by Butler et al. (2008) and Han et al. (2012); to Hypsilophodontidae by Gilmore (1915), Lull (1924), Parks (1926), Parks (1926), Hay (1930), Russell (1930), Parks (1935), Swinton (1936), Romer (1956), Parsch (1963), Kuhn (1964), Estes (1964), Simmons (1965), Romer (1966), Charig (1967), Thulborn (1970), Swinton (1970), Thulborn (1970), Thulborn (1971), Wolberg (1980), McIntosh (1981), Weishampel and Weishampel (1983), Milner and Norman (1984), Norman (1984), Breithaupt (1985), Cooper (1985), Clemens (1986), Carroll (1988), Galton (1989), Sues and Norman (1990), Grigorescu (1992), Weishampel and Heinrich (1992), Galton (1995), Sues (1997), Ryan (1997), Dodson (1997), Winkler et al. (1997), Eberth et al. (2001), Ryan and Evans (2005) and Alifanov and Saveliev (2014); to Thescelosaurinae by Sternberg (1940), Brown and Druckenmiller (2011), Brown et al. (2013) and Madzia et al. (2018); to Thescelosauridae by Sternberg (1937), Tatarinov (1964), Rozhdestvensky and Tatarinov (1964), Thulborn (1974), Eberth (1997) and Herne et al. (2019); and to Clypeodonta by Dieudonné et al. (2020).