Brachiosaurus was named by Riggs (1903). Its type is Brachiosaurus altithorax.

It was synonymized subjectively with Astrodon by Kingham (1962).

It was assigned to Sauropoda by Janensch (1914); to Cetiosauridae by Huene (1908), Huene (1908), Huene (1909), von Zittel (1911) and Lull (1924); to Sauropoda by Joleaud (1922) and Huene (1958); to Brachyosauridae by Langston (1974); to Brachiosaurinae by Huene (1927), Huene (1927), Huene (1929), Romer (1956), Steel (1970), Swinton (1970), Coombs and Molnar (1981), Santafé et al. (1982) and Sanz (1986); to Camarasauridae by Dong (1998); to Titanosauriformes by Salgado et al. (1997), Smith et al. (2001), Wilson (2002), Apesteguía (2005), Apesteguía (2005), Curry Rogers (2005), Allain and Aquesbi (2008), Suteethorn et al. (2010), Whitlock (2011), Sekiya (2011), Mannion and Calvo (2011), Filippi et al. (2011) and Xing et al. (2015); and to Brachiosauridae by Riggs (1904), Williston (1905), Lull (1919), Nopcsa (1928), Hay (1930), Lapparent (1943), Lapparent (1960), Tatarinov (1964), Romer (1966), Galton (1977), Madsen and Miller (1979), Molnar (1980), Russell (1984), Bonaparte (1986), Bonaparte (1986), Carroll (1988), McIntosh (1990), Miller et al. (1991), Salgado (1993), Salgado and Calvo (1993), Britt and Naylor (1994), Upchurch (1995), Upchurch (1995), Jacobs et al. (1996), McIntosh (1997), Dodson (1997), Sereno (1997), Sereno (1998), Wilson and Sereno (1998), Bonaparte (1999), Wedel et al. (2000), Maxwell and Cifelli (2000), Thulborn (2002), Upchurch et al. (2004), Royo-Torres et al. (2006), Chure et al. (2006), Moser et al. (2006), Rose (2007), Taylor (2009), You and Li (2009), Ksepka and Norell (2010), Taquet (2010), D'Emic (2012), Mannion et al. (2013), D'Emic (2013), Carballido et al. (2015), D'Emic et al. (2016), Salisbury et al. (2016), Mannion et al. (2017) and Mannion et al. (2019).