Ankylosaurus was named by Brown (1908). It is the type genus of Ankylosauridae, Ankylosaurinae, Ankylosaurini. It was considered monophyletic by Vickaryous et al. (2004).

It was synonymized subjectively with Stereocephalus by von Zittel (1911); it was synonymized subjectively with Euoplocephalus by Romer (1956), Parsch (1963) and Romer (1966).

It was assigned to Stegosauria by Sternberg (1909); to Stegosauridae by Joleaud (1922); to Acanthopholidae by Huene (1927); to Nodosauridae by Wieland (1911), Hennig (1915), Nopcsa (1915), Lull (1921), Lull (1924), Nopcsa (1928), Russell (1930), Gilmore (1933), Parks (1935), Young (1935), Tatarinov (1964), Kuhn (1964) and Simmons (1965); to Ankylosaurinae by Nopcsa (1929), Kuhn (1966), Tumanova (1987), Sereno (1998), Ford (2000), Vickaryous et al. (2001), Vickaryous et al. (2004), Han et al. (2014), Arbour et al. (2014) and Yang et al. (2017); to Ankylosaurini by Arbour and Currie (2016) and Arbour and Mallon (2017); and to Ankylosauridae by Brown (1908), Huene (1909), Brown (1914), Lambe (1918), Nopcsa (1928), Hay (1930), Maleev (1954), Maleev (1956), Swinton (1970), Coombs (1978), Coombs (1978), Coombs (1979), Russell (1984), Breithaupt (1985), Sereno (1986), Clemens (1986), Coombs (1986), Carroll (1988), Coombs and Deméré (1996), Eberth (1997), Dodson (1997), Kirkland (1998), Eberth et al. (2001), Carpenter (2001), Pereda Suberbiola and Galton (2001), Ford and Kirkland (2001), Garcia and Pereda-Suberbiola (2003), Carpenter (2004), Lü et al. (2007), Lü et al. (2007), Burns (2007), Carpenter et al. (2008), Burns (2008), Osi and Makádi (2009), Parsons and Parsons (2009), Arbour et al. (2014) and Galton (2019).