Janssen is an ancient impact crater located in the highland region near the southeastern lunar limb.

The wall forms a distinctive hexagonal shape upon the rugged lunar surface, with a slight curvature at the vertices.

In the southern two-thirds of Janssen can be discerned the remains of a large, concentric crater, the wall of which is overlaid by Fabricius.

The rille curves from the rim of Fabricius to the southeast of the outer wall of Janssen, extending for a distance of up to 140 kilometers.

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