Mersenius is a lunar crater that is located to the west of the Mare Humorum, on the southwest part of the Moon. To the southwest is the Cavendish crater, and to the south-southeast lies Liebig crater.

The rim of Mersenius is heavily worn, especially in the low northern section. The crater 'Mersenius N' lies across the southwest rim. The interior has been flooded by basaltic-lava, which bludges upward forming a convex domed shape. This was most likely formed by lava upwelling beneath the surface, perhaps comping from the nearby mare. There are several tiny craterlets across the floor surface, but little in the way of a central peak.

To the east of the crater on the surface and edges of the Mare Humorum is a rille system designated Rimae Mersenius. These rilles are generally parallel and run to the north-northeast for a length of about 230 kilometers.

Satellite craters

By convention these features are identified on Lunar maps by placing the letter on the side of the crater mid-point that is closest to Mersenius crater.