William Lassell was an English amateur astronomer who discovered Neptune's moon Triton (1846), Saturn's moon Hyperion (1848), and Uranus's moons Ariel and Umbriel (1851).

Born in Bolton, Lancashire, Lassell was a successful brewer before turning to astronomy. He set up a private observatory at Starfield near Liverpool and developed an interest in techniques for building very large reflectors. In 1845 he completed a 24-inch (0.6-meter) reflector, which was the first sizeable reflector on an equatorial mount, and used it to discover Neptune's largest moon – just 17 days after Neptune itself was found. Lassell also discovered Hyperion and the crêpe ring of Saturn, independently of William Bond. In 1851 he discovered Ariel and Umbriel. In the winter of 1852–1853, he moved his 24-inch telescope to Malta, where, under much clearer skies than were available in industrial England he was able to observe and catalogue hundreds of new nebulae.