William Lassell (June 18 1799 – October 5 1880) was a British astronomer.

He made his fortune as a beer brewer, which enabled him to indulge his interest in astronomy. He built an observatory near Liverpool with a 24-inch reflector telescope, for which he pioneered the use of an equatorial mount for easy tracking of objects as the earth rotates. He ground and polished the mirror himself, using equipment he constructed himself.

In 1846 he discovered Triton, the largest moon of Neptune, soon after the discovery of Neptune itself. In 1848 he independently co-discovered Hyperion, a moon of Saturn. In 1851 he discovered Ariel and Umbriel, two new moons of Uranus.

When Queen Victoria visited Liverpool in 1851, Lassell was the only local she specifically requested to meet.

In 1855, he built a 48-inch telescope, which he installed in Malta because of the better observing conditions (weather) compared to Britain.

Upon his death, he left a fortune of £80,000 (equivalent of multi-millionaire by today's standards).

He won the Gold Medal of the Royal Astronomical Society in 1849, and served as its president for two years starting in 1870.

Craters on Mars and on the Moon, and a ring of Neptune, were named in his honor.