Nielsen is a lunar impact crater on the Oceanus Procellarum.
It is located north-east of Montes Agricola on the western hemisphere of the Moon.
Nielsen is a bowl-shaped formation that lies astride a small ridge that runs north-northwestwards towards the Mons Rümker.
The latter is an unusual raised formation of lunar domes.
In 1973, the crater was named jointly after the Danish astronomer Axel Nielsen (1902-1970) and the Danish–American physicist Harald Herborg Nielsen (1903-1973), by the IAU.