Polacanthus foxii was named by Grabau (1921). Its type specimen is BMNH R.175, a partial skeleton (scapulocoracoid), and it is a 3D body fossil. Its type locality is east of Barne's Chine (BMNH), which is in a Barremian marine claystone in the Wessex Formation of the United Kingdom. It was considered monophyletic by Vickaryous et al. (2004).

It was corrected as Polacanthus foxi by Anonymous (1932) and Madsen and Miller (1979); it was misspelled as Polacanthus foxi by Jackson (1933), Swinton (1934), Swinton (1936), Maleev (1954), Maleev (1956), Tatarinov (1964), Kuhn (1964), Bodily (1970), Swinton (1970), Blows (1978), Blows (1982), Galton (1983), Blows (1983), Blows (1987), Hutt et al. (1989), Cheevely and Chapman (1992), Pereda-Suberbiola (1993), Molnar and Wiffen (1994), Pereda-Suberbiola (1994), Blows (1996), Norman (1997), Vickaryous and Ryan (1997), Knoll et al. (1998), Blows (1998), Pereda Suberbiola and Barrett (1999), Pereda Suberbiola et al. (1999), Ford (2000), Naish and Martill (2001), Pereda Suberbiola and Galton (2001), Carpenter (2001), Blows (2001), Carpenter et al. (2001), Ford and Kirkland (2001), Ruiz-Omeñaca and Canudo (2003), Garcia and Pereda-Suberbiola (2003), Vickaryous et al. (2004), Norman (2004), Naish and Martill (2007), Naish and Martill (2008), Galton (2009), Barrett and Maidment (2011), Kirkland and Loewen (2011), Gasulla et al. (2011), Yang et al. (2013), Kirkland et al. (2013), Han et al. (2014), Borinder et al. (2016), Kirkland et al. (2016) and Kineer et al. (2016); it was considered a nomen dubium by Galton and Ayyasami (2017).