Niels Johan Føyn, original surname Nielsen (February 23, 1860 – June 13, 1945) was a Norwegian meteorologist.
Føyn was born in Slagen in the municipality of Tønsberg, Norway.
He was the son of the ship operator Peder Nielsen (1818–1904) and he later adopted the surname Føyn from the oeconym of the farm where his family lived.
He received his candidatus realium degree in 1886 and then worked as an assistant at the Norwegian Meteorological Institute.
He received a state scholarship to study in Berlin and Vienna from 1893 to 1895.
From 1903 to 1915, Føyn was the supervisor of the Bergen Meteorological Observatory, and from 1915 to 1928 he was the deputy director of the Norwegian Meteorological Institute.
He was made a member of the Norwegian Academy of Science and Letters in 1904.