We determine photometric metal abundance estimates for individual main-sequence stars in the Virgo Overdensity (VOD), which covers almost 1000 deg^2 on the sky, based on a calibration of the metallicity sensitivity of stellar isochrones in the gri filter passbands using field stars with well-determined spectroscopic metal abundances. Despite the low precision of the method for individual stars, we derive [Fe/H] = -2.0 +/-0.1 (internal) +/-0.5 (systematic) for the metal abundance of the VOD from photometric measurements of 0.7 million stars in the Northern Galactic hemisphere with heliocentric distances from ~10 kpc to ~20 kpc. The metallicity of the VOD is indistinguishable, within Delta [Fe/H] < 0.2, from that of field halo stars covering the same distance range. This initial application suggests that the SDSS gri passbands can be used to probe the properties of main-sequence stars beyond ~10 kpc, complementing studies of nearby stars from more metallicity-sensitive color indices that involve the u passband.
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