Cetotherium (Plesiocetopsis) was named by Brandt (1873) [Sepkoski's age data: T Mi-u T Plio-l]. It was considered monophyletic by Canto et al. (2010).

It was synonymized subjectively with Cetotherium by Lydekker (1887); it was Plesiocetus (Plesiocetopsis) by True (1912); it was synonymized subjectively with Plesiocetus by McKenna and Bell (1997); it was reranked as Plesiocetopsis by Kellogg (1928), Hay (1930), Kellogg (1931), Simpson (1945), Carroll (1988), Sepkoski (2002), Whitmore and Barnes (2008), Canto et al. (2010), Steeman (2010), Gol'din and Steeman (2015) and Berta (2017).

It was assigned to Cetotherium by Brandt (1873), Trouessart (1904) and Palmer (1904); to Plesiocetus by True (1912); to Physeteridae by Carroll (1988); to Cetacea by Sepkoski (2002); to Diorocetidae by Steeman (2010); to Cetotheriidae by Kellogg (1928), Hay (1930), Kellogg (1931), Simpson (1945), Whitmore and Barnes (2008) and Canto et al. (2010); and to Tranatocetidae by Gol'din and Steeman (2015) and Berta (2017).