Hagen's pit viper inhabits primary lowland and hill forest up to elevations of 600 metres in parts of southern Thailand, Peninsular Malaysia, the Indonesian island of Sumatra and other nearby islands such as Pulau Bangka.

It is nocturnal in habits and is typically encountered on low vegetation where it hunts for small mammals and birds.

Its body is robust, and its head is of typical pit viper shape, being markedly wider that its body. Its body colour is green, rarely with vague slightly darker banding. Many specimens bear a series of small, pale spots either side of the vertebral line (with each spot not greater than two body scales).

The underside is pale green and a continuous white line, which commences behind the eye and extends down to the base of the tail, separates ventral scales from the dorsal scales. The chin and throat, and scales above and below the lips are pale green to whitish. The tail is pinkish or reddish brown.

In Singapore the species is listed as 'indeterminate' i.e. there are no unequivocal records from the country.