Neptune was supposedly discovered in 1846 by Johann Gottfried Galle using calculations by Urbain Le Verrier and John Couch Adams, making it a joint British-French-German discovery.
But these astronomers were not the first to observe Neptune. That honor goes to the famous Italian astronomer Galileo Galilei.
While sketching the moons of Jupiter with his newly discovered telescope, Galileo twice drew Neptune, which happened to be in conjunction with Jupiter in early 1613. It's usually said that Galileo mistook Neptune for a star because of its slow movement.