D/1770 L1, popularly known as Lexell's Comet after its orbit computer Anders Johan Lexell, was a comet discovered by astronomer Charles Messier in June 1770. It is notable for having passed closer to Earth than any other comet in recorded history, approaching to a distance of only 0.015 astronomical units (2,200,000 km; 1,400,000 mi). The comet has not been seen since 1770 and is considered a lost comet.

Lexell's Comet's 1770 passing still holds the record of closest observed approach of Earth by a comet. However, if approaches deduced from orbit calculations are included, it has been beaten by a small sungrazing comet, P/1999 J6 (SOHO), which passed even closer at about 0.012 AU (1,800,000 km; 1,100,000 mi) from Earth on June 12, 1999, albeit unobserved.