[3] Despite being a predominantly icy body more than 400 km (250 mi) in diameter, Proteus's shape deviates significantly from an ellipsoid.

[13] Proteus was discovered from the images taken by the Voyager 2 space probe two months before its Neptune flyby in August 1989.

Proteus orbits Neptune at a distance of approximately 117,647 km (73,102 mi), nearly equal to 4.75 times the equatorial radius of the planet.

It was not discovered by Earth-based telescopes because Proteus orbits so close to Neptune that it is lost in the glare of reflected sunlight.

[15] In the near-infrared range of around the wavelength of 2 μm, Proteus's surface becomes less reflective, suggesting the presence of complex organic compounds such as hydrocarbons or cyanides.

While Proteus is usually thought to contain significant amounts of water ice, there is no definite spectroscopic confirmation of its presence on the surface.

[12] However, James Webb Space Telescope NIRCam spectrophotometry of the moon has revealed a 3 μm absorption feature, indicating possible water ice or hydrated minerals on the surface of Proteus.

[18] Proteus's infrared albedo measured at 1.4, 2.1, 3.0, and 4.6 microns appears to match other dark Solar System small bodies, such as 2004 EW95.

[6] Proteus is slightly elongated in the direction of Neptune, although its overall shape is closer to an irregular polyhedron than to a triaxial ellipsoid.

[19] Proteus, like the other inner moons of Neptune, is unlikely to be an original body that formed with it, and is more likely to have accreted from the rubble that was produced after Triton's capture.

At the time of the formation of Proteus, its orbit was estimated to be 8,000 km (5,000 mi) less than its current distance from Neptune.