Arp 166 is a pair of interacting elliptical galaxies approximately 225 million light-years away from Earth in the constellation of Triangulum.

[4] Arp 166 was discovered by German-born British astronomer William Herschel on September 12, 1784, but he did not resolve this close pair of galaxies, therefore he described it as a single object NGC 750.

[4][3] Arp 166 was first seen as a double by Irish engineer and astronomer Bindon Stoney on October 11, 1850, who used Lord Rosse's 72" telescope.

[4] At least 100,000,000 years have passed since the moment of the first strong tidal perturbation between these two galaxies.

[5] A large, diffuse tidal tail extends 20 arcsec (10 kpc) to the north-east of the pair.