Ursa Major III (UMa III) is a dwarf satellite galaxy of the Milky Way, the smallest and faintest ever discovered.^[2] It was found by the deep, wide field Ultraviolet Near Infrared Optical Northern Survey (UNIONS), a collaboration between the Canada–France–Hawaii Telescope and Pan-STARRS (two observatories in Hawaii), with additional data provided by the Keck Observatory's Deep Imaging Multi-Object Spectrograph (DEIMOS), which has 64 megapixels of resolution.

Ursa Major III's discovery was announced in November 2023, with a paper appearing in The Astrophysical Journal in January 2024. It contains a metal-poor stellar population, indicating an extreme age of 11 billion years. Located about 32,600 light years away, it has a diameter of just 19.6 light years and is thought to contain only about 60 stars. Combined with its absolute magnitude of only +2.2, this makes it by far the Milky Way's dimmest satellite, and only about as bright as Altair. This absolute magnitude corresponds to a total luminosity of 11.4 L_☉.^[1]

Ursa Major III is predicted to have a mass-to-light ratio of about 6,500. However, this becomes only 1,900 with the removal of one of the stars suspected to be part of the galaxy. This very high value may indicate the presence of a massive dark matter halo, suggesting that Ursa Major III could indeed be a true dwarf galaxy, albeit one with an extremely low stellar mass.^[1] Ursa Major III's total stellar mass is only about 16 M_☉, making it the least massive Milky Way satellite known and by far the least massive galaxy known, if it is proven to truly be a galaxy in future research.