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. To the east-southeast is the crater Wollaston.
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.
The crater is named jointly after the Danish astronomer Axel Nielsen (1902-1970) and the Danish–American physicist Harald Herborg Nielsen (1903-1973).