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We report the discovery of Tucana B, an isolated ultra-faint dwarf galaxy ata distance of D=1.4 Mpc. Tucana B was found during a search for ultra-faintsatellite companions to the known dwarfs in the outskirts of the Local Group,although its sky position and distance indicate the nearest galaxy to be$\sim$500 kpc distant. Deep ground-based imaging resolves Tucana B into stars,and it displays a sparse red giant branch consistent with an old, metal poorstellar population analogous to that seen in the ultra-faint dwarf galaxies ofthe Milky Way, albeit at fainter apparent magnitudes. Tucana B has a half-lightradius of 80$\pm$40 pc, and an absolute magnitude of$M_V$=$-$6.9$^{+0.5}_{-0.6}$ mag ($L_V$=$(5^{+4}_{-2})\times$10$^4$$L_{\odot}$), which is again comparable to the Milky Way's ultra-faintsatellites. There is no evidence for a population of young stars, either in theoptical color magnitude diagram or in GALEX archival ultraviolet imaging, withthe GALEX data indicating $\log (\mathrm{SFR_{NUV}/M_\odot \, yr^{-1}}) < -5.4$for star formation on $\lesssim$100 Myr time scales. Given its isolation andphysical properties, Tucana B may be a definitive example of an ultra-faintdwarf that has been quenched by reionization, providing strong confirmation ofa key driver of galaxy formation and evolution at the lowest mass scales. Italso signals a new era of ultra-faint dwarf galaxy discovery at the extremeedges of the Local Group.

Tucana B: A Potentially Isolated and Quenched Ultra-faint Dwarf Galaxy at D$\approx$1.4 Mpc