Cetiosaurus was named by Owen (1841). It is the type genus of Cetiosauridae, 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 Sauropoda by Gadow (1901); to Atlantosauridae by Nopcsa (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 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) and Taquet (2010); to Eusauropoda by Ksepka and Norell (2010), Sekiya (2011) and Otero and Pol (2013); and to Gravisauria by Rauhut et al. (2020).