Haplocanthosaurus was named by Hatcher (1903). Its type is Haplocanthus priscus. It is the type genus of Haplocanthosauridae.

It was assigned to Morosauridae by Hatcher (1903); to Camarasaurinae by Nopcsa (1928); to Cardiodontinae by Huene (1927), Huene (1927), Huene (1929); to Sauropoda by Tatarinov (1964); to Brachiosauridae by Riggs (1904), Williston (1905), Lull (1919), Romer (1966), Madsen and Miller (1979); to Cetiosaurinae by Romer (1956), Steel (1970), Coombs and Molnar (1981); to Dicraeosauridae by Bonaparte (1986); to Cetiosaurinae by Kuhn (1939), McIntosh (1990); to Sauropoda by Upchurch (1997); to Haplocanthosauridae by Bonaparte (1999); to Camarasauridae by Hay (1930), Upchurch (1995), Upchurch (1995), Upchurch et al. (2004), Zhang et al. (2009); to Neosauropoda by Canudo et al. (2010); to Macronaria by Sereno (1997), Sereno (1998), Wilson and Sereno (1998), Sereno (1999), de Almeida et al. (2004), Gallina and Apesteguía (2005), Royo-Torres et al. (2006), Carballido et al. (2011); to Diplodocoidea by Wilson (2002), Salgado et al. (2006), Remes (2006), Suteethorn et al. (2010), Ksepka and Norell (2010), Whitlock (2011); to Cetiosauridae by Huene (1908), Huene (1908), Huene (1909), von Zittel (1911), Lull (1924), Lapparent (1943), Anonymous (1959), Galton (1977), Molnar (1980), McIntosh (1981), Russell (1984), Carroll (1988), McIntosh (1990), Britt and Naylor (1994), McIntosh (1997), Dodson (1997), Chure et al. (2006), Erickson (2014); to Eusauropoda by Xing et al. (2015); and to Diplodocoidea by Tschopp et al. (2015), Mannion et al. (2019), Boisvert et al. (2025).