Alamosaurus was named by Gilmore (1922).

It was assigned to Sauropoda by Gilmore (1922); to Diplodocidae by Hay (1930); to Atlantosauridae by Russell (1930); to Sauropoda by Lull (1924), Tatarinov (1964); to Cetiosauridae by Powell (1973); to Antarctosauridae by Breithaupt (1985); to Titanosaurinae by Romer (1956), Charig (1967), Steel (1970), Kues et al. (1980), Casanovas et al. (1987); to Titanosauroidea by Upchurch (1997); to Titanosauria by Wilson and Sereno (1998); to Opisthocoelicaudiinae by Wilson (2002), You et al. (2004), Curry Rogers (2005), Calvo et al. (2007), González Riga et al. (2009), Suteethorn et al. (2010); to Eutitanosauria by Powell (2003), Filippi et al. (2011); to Titanosauridae by Huene (1927), Huene (1927), Huene (1927), Huene (1929), Huene (1929), Stromer (1932), Huene (1941), Gilmore (1946), Romer (1966), Molnar (1980), Lehman (1981), Bonaparte (1984), Russell (1984), Carroll (1988), Lucas and Hunt (1989), McIntosh (1990), McIntosh (1990), Le Loeuff (1991), Currie (1992), Upchurch (1995), Upchurch (1995), Jacobs et al. (1996), Salgado (1996), Novas (1997), McIntosh (1997), Ryan (1997), Dodson (1997), Salgado and Calvo (1997), Salgado et al. (1997), Prasad and Sahni (1999), Maxwell and Cifelli (2000), Smith et al. (2001), Calvo and González Riga (2003), Apesteguía (2005), Moser et al. (2006), Jasinski et al. (2011); to Opisthocoelicaudinae by Calvo et al. (2007), González Riga and Ortiz David (2014); to Somphospondyli by Xing et al. (2015); to Lithostrotia by Upchurch et al. (2004), Ksepka and Norell (2010), Mocho et al. (2014), Filippi et al. (2019); to Saltasauridae by Wilson and Upchurch (2003), Wilson and Mohabey (2006), Zaher et al. (2011), Coria et al. (2013), Mannion et al. (2013), Poropat et al. (2016), González Riga et al. (2016), Páramo et al. (2020); and to Saltasaurinae by D'Emic (2012), Navarro et al. (2022).