Trimeresurus hageni, commonly known as Hagen's pit viper[4] and Hagen's green pit viper,[1] is a species of venomous snake in the subfamily Crotalinae of the family Viperidae.

[5] The specific name, hageni, is in honor of German naturalist Dr. Bernhard Hagen, who collected mammals, birds, reptiles, and insects in the eastern part of Sumatra.

[4] The lectotype has a total length (including tail) of 97 cm (38 in).

[3] T. hageni is found in Peninsular Thailand, Peninsular Malaysia, East Malaysia (Borneo), and Indonesia (Sumatra and the nearby islands of Bangka, Simalur, Nias, Batu, and the Mentawai Islands).

[2] The preferred natural habitat of T. hageni is forest, at altitudes of 100–300 m (330–980 ft).