Telmatosaurus was named by Nopcsa (1903).

It was synonymized subjectively with Orthomerus by Nopcsa (1915), Nopcsa (1915), Nopcsa (1915), Nopcsa (1915), Nopcsa (1923), Huene (1927), Romer (1956), Tatarinov (1964), Kuhn (1964) and Romer (1966).

It was assigned to Trachodontidae by Huene (1909), von Zittel (1911), Matthew (1915) and Gilmore (1922); to Hadrosaurinae by Brett-Surman (1979), Weishampel and Weishampel (1983), Brinkmann (1988) and Chapman and Brett-Surman (1990); to Iguanodontia by Forster (1997); to Euhadrosauria by Weishampel et al. (2010); to Hadrosauroidea by Godefroit et al. (1998), You et al. (2003), Godefroit et al. (2004), Prieto-Márquez et al. (2006), Godefroit et al. (2008), Sues and Averianov (2009), McDonald et al. (2010), Prieto-Marquez and Salinas (2010), Prieto-Márquez and Wagner (2010) and Ramírez-Velasco et al. (2012); to Hadrosauridae by Nopcsa (1903), Charig (1967), Weishampel and Horner (1990), Weishampel et al. (1991), Horner (1992), Weishampel et al. (1993), Le Loeuff and Buffetaut (1995), Buffetaut (1997), Pereda Suberbiola and Galton (1997), Casanovas-Cladellas et al. (1998), Casanovas et al. (1999), Vremir and Codrea (2002), Grigorescu (2003), Prieto-Marquez et al. (2003), Horner et al. (2004), Lund and Gates (2006), Pereda Suberbiola and Galton (2007), Pereda Suberbiola and Galton (2007), Pereda-Suberbiola et al. (2009), Cruzado-Caballero et al. (2010), Grigorescu (2010) and Norman (2014); to Hadrosauroidea by Prieto-Márquez (2010), Prieto-Márquez (2010), Juárez Valieri et al. (2010), Wang et al. (2011), McDonald (2012), Brusatte et al. (2013), Blanco et al. (2015), Xu et al. (2016), Cruzado-Caballero and Powell (2017) and Dalla Vecchia (2020); and to Hadrosauridae by Verdú et al. (2021).