Jansen is a lunar impact crater in the north part of the Mare Tranquillitatis.

It was named after 17th century Dutch optician Zacharias Janssen.

The rim of Jansen is low and narrow, with a notch along the western edge.

To the northwest of the rim is a rille named Rima Jansen, and to the east are wrinkle ridges in the mare surface.

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