We present the discovery of a unique interstellar structure: a thirty-degree long, thin, circular arc of ultraviolet emission centered on Galactic coordinates, (l,b)=(107.7,+60.0), found in both the near- and far-ultraviolet channels of the Galaxy Evolution Explorer (GALEX) All-Sky Imaging Survey. This “Ursa Major Arc” traces approximately one-sixth of a full circle of angular diameter 60 degrees, and is nearly perfectly circular; the standard deviation from a circular fit for most of the structure is 0.18 degrees, or 0.6% of the angular radius. An extrapolation of this arc to a full circle covers 27% of the sky above Galactic latitude b=32 degrees. This circle is centered on the “handle” of the “Big Dipper”, crosses through the “Little Dipper”, and completely encircles this asterism. We show that the near UV/far UV ratio is consistent with this emission arising from two-photon hydrogen recombination in a shock front. Associated Hα emission has been detected by the MDW Hα survey conduted by a team of amateur astronomers. We propose that this structure was produced by a nearby supernova explosion—within a few hundred parsecs—and compare it to models of a supernova blast wave. We then demonstrate how diffuse Hα observations and ultraviolet absorption line observations may be used to further constrain its evolutionary state, and explore its significance as a Galactic “foreground” for extragalactic and cosmological investigations.