Nemegtosaurus was named by Nowinski (1971). It is not extant.

It was assigned to Atlantosauridae by Osmólska et al. (1972), Dong (1977), Bonaparte (1978) and Kielan-Jaworowska (1984); to Titanosaurinae by Dong (1992); to Dicraeosauridae by Hunt et al. (1995); to Diplodocidae by Berman and McIntosh (1978), Charig (1980), Carroll (1988), Mcintosh et al. (1992) and McIntosh (1997); to Dicraeosaurinae by Nowinski (1971), Kurzanov and Bannikov (1983), Kurzanov and Bannikov (1983), McIntosh (1990), McIntosh (1990), Currie (1992) and Maryanska (2000); to Titanosauriformes by Maxwell and Cifelli (2000); to Titanosauridae by Rozhdestvensky (1977), Chatterjee and Rudra (1996), Salgado and Calvo (1997), Dong (1998) and Currie et al. (2003); to Lithostrotia by Curry Rogers (2005); to Pakisauridae by Malkani (2008); to Titanosauria by Wilson and Sereno (1998), You et al. (2004), Wilson (2004) and Wilson and Upchurch (2009); to Somphospondyli by Xing et al. (2015); to Nemegtosauridae by Upchurch (1995), Upchurch (1995), Wilson (2002), Buffetaut et al. (2002), Upchurch et al. (2004), Wilson (2005), Candeiro et al. (2006), Suteethorn et al. (2010), Ksepka and Norell (2010), Zaher et al. (2011), Grellet-Tinner et al. (2011), Grellet-Tinner et al. (2012) and Poropat et al. (2016); and to Opisthocoelicaudiinae by Navarro et al. (2022).