Apatosaurus alenquerensis was named by Lapparent and Zbyszewski (1957). Its type specimen is MGIGM 4956 and is a 3D body fossil. Its type locality is Moinho do Carmo, Alenquer, which is in a Kimmeridgian terrestrial claystone in the Lourinhã Formation of Portugal.

It was recombined as Atlantosaurus alenquerensis by Charig (1967), Steel (1970); it was recombined as Camarasaurus alenquerensis by McIntosh (1990), McIntosh (1990), McIntosh et al. (1996), Wilson and Sereno (1998); it was recombined as Lourinhasaurus alenquerensis by Dantas et al. (1998), Upchurch et al. (2004), Mateus (2006), Royo-Torres et al. (2006), Royo-Torres et al. (2007), Escaso et al. (2007), Luque et al. (2008), Ortega et al. (2009), Canudo et al. (2010), Ortega et al. (2010), Mateus and Milàn (2010), Mocho et al. (2013), Mocho et al. (2014), Cobos et al. (2014), Wings (2015), Tschopp et al. (2015), Mocho et al. (2016), Mocho et al. (2017), Tschopp et al. (2019), Costa and Mateus (2019), Mocho et al. (2019).