Proteus was discovered in 1989 by Voyager 2. Though it is larger than Nereid, it was not discovered earlier because it is very dark and so close to Neptune that it is difficult to see in the glare of the much brighter planet.

Proteus is very irregularly shaped. The satellite is about as big as an irregularly shaped body can ever be, before being pulled to a more regular shape by it's own gravity.

Proteus is heavily cratered, but shows no signs of volcanic activity.