Messier 66 or M66, also known as NGC 3627, is an intermediate spiral galaxy in the southern, equatorial half of Leo.

It was discovered by French astronomer Charles Messier[8] on 1 March 1780, who described it as "very long and very faint".

[9] M66 has a morphological classification of SABb,[5] indicating a spiral shape with a weak bar feature and loosely wound arms.

Gravitational interaction from its past encounter with neighboring NGC 3628 has resulted in an extremely high central mass concentration; a high molecular to atomic mass ratio; and a resolved non-rotating clump of H I material apparently removed from one of the spiral arms.

The latter feature shows up visually as an extremely prominent and unusual spiral arm and dust lane structures as originally noted in the Atlas of Peculiar Galaxies.