Niels Nielsen may refer to:
- Niels Nielsen (sailor) (1883–1961), Norwegian Olympic silver medalist in 1920
- Niels Kristian Nielsen (1897–1972), Danish gymnast at the 1920 Summer Olympics
- Niels Turin Nielsen (1887–1964), Danish gymnast at the 1908 and 1920 Summer Olympics
- Niels Erik Nielsen (1893–1974), Danish gymnast at the 1920 Summer Olympics
- Niels Nielsen (mathematician) (1865–1931), Danish mathematician
- Niels Nielsen, caretaker of the Old Man of the Mountain