Mersenius is a lunar impact crater that is located to the west of the Mare Humorum, in the southwestern part of the Moon.

[1] The rim of Mersenius is heavily worn, especially in the low northern section.

The interior has been flooded by basaltic lava, which bulges upwards forming a convex domed shape with an estimated height of 450 metres relative to the floor edges.

[2][3] To the east of the crater on the surface and edges of the Mare Humorum is a rille system designated Rimae Mersenius.

[4] By convention these features are identified on lunar maps by placing the letter on the side of the crater midpoint that is closest to Mersenius.