Reiner is a lunar impact crater on the Oceanus Procellarum, in the western part of the Moon. It has a nearly circular rim, but appears oval in shape due to foreshortening. The rim edge is well-defined and has not been eroded by impacts. In the midpoint of the irregular crater floor is a central peak. Outside the rim is a hummocky that extends out across the mare for about half a crater diameter.
To the west-northwest of the crater on the Oceanus Procellarum is the unusual feature Reiner Gamma, a fish-shaped surface marking of ray-like material with a high albedo.
Reiner is a crater of Eratosthenian age. It is named after the astronomer Vincentio Reinieri, a disciple of Galileo Galilei.
By convention these features are identified on lunar maps by placing the letter on the side of the crater midpoint that is closest to Reiner.