Cetiosaurus was named by Owen (1841). It is the type genus of Cetiosaurinae.

It was synonymized subjectively with Cardiodon by Marsh (1895), Marsh (1896), Marsh (1898), Sauvage (1900), Lydekker (1904) and Sauvage (1906).

It was assigned to Saurii by Meyer (1845); to Crocodilidae by d'Orbigny (1849); to Sauria by Quenstedt (1852); to Crocodilia by Owen (1842), Owen (1854) and Owen (1859); to Opisthocoelia by Owen (1859), Owen (1860) and Owen (1861); to Dinosaurii by Quenstedt (1867); to Dinosauria by Lydekker (1877); to Atlantosauridae by Nopcsa (1901); to Sauropoda by Gadow (1901); to Morosauridae by Dollo (1884) and Hatcher (1903); to Sauropoda by Williston (1878), Winkler (1893) and Joleaud (1922); to Camarasaurinae by Nopcsa (1928); to Cardiodontinae by Huene (1927), Huene (1927) and Huene (1929); to Brachiosauridae by Romer (1966); to Cetiosaurinae by Romer (1956), Steel (1970), Swinton (1970), Kaever and Lapparent (1974) and Coombs and Molnar (1981); to Cetiosaurinae by Kuhn (1939) and McIntosh (1990); to Sauropoda by Quenstedt (1882), Sauvage (1882), Marsh (1884), Marsh (1885) and Upchurch (1997); to Neosauropoda by Wilson (2002); to Eusauropoda by Ksepka and Norell (2010), Sekiya (2011) and Otero and Pol (2013); to Cetiosauridae by Lydekker (1888), Zittel (1890), Baur (1891), Depéret (1896), Mansel-Pleydell (1902), Huene (1908), Huene (1908), Arldt (1909), Huene (1909), von Zittel (1911), Lull (1924), Lapparent (1943), Huene (1944), Bataller (1960), Delair (1960), Tatarinov (1964), Delair (1973), Molnar (1980), Taquet (1984), Bonaparte (1986), Bonaparte (1986), Carroll (1988), McIntosh (1990), Mcintosh et al. (1992), Bonaparte and Coria (1993), Upchurch (1995), Upchurch (1995), McIntosh (1997), Smith (1997), Dong (1998), Upchurch and Martin (2002), Upchurch and Martin (2003), Upchurch et al. (2004), Barrett (2006), Naish and Martill (2007), Taquet (2010) and Ibrahim et al. (2017); and to Gravisauria by Rauhut et al. (2020).