NGC 3646

NGC 3646 is a galaxy in the Leo constellation that has variously been described as "a strange spiral galaxy" of morphological classication Sc or SAa, or as "a ring-shaped galaxy".

NGC 3646 has a structure consisting of three parts: an inner hub, a rapidly rotating spiral, and an irregular outer ring surrounding the spiral with an angular feature at one point of the ring. Although estimates vary for its distance, the NASA/IPAC Extragalactic Database lists its distance as 67.49 Mpc, ±4.74 Mpc.

Burbidge et al. estimated that their measurements of motion in the outer ring were not consistent with stable circular orbits. Afanas'ev et al. argue that this conclusion was erroneous, based on incorrect measurements. Instead, they find a galaxy rotation curve that "places the galaxy among the most rapidly rotating and massive systems", "one of the largest and most luminous spirals in the local universe".

NGC 3646 forms an isolated pair with a smaller companion galaxy, NGC 3649. The high rate of star formation in NGC 3646, the low rate in its companion, and the unusual shape of the outer ring in NGC 3646 may have resulted from interactions between these two galaxies.

Two supernovae have been seen in NGC 3646: SN 1989N, and SN 1999cd.