We utilize Sloan Digital Sky Survey imaging and spectroscop y f ∼53,000 star-forming galaxies at z ∼ 0.1 to study the relation between stellar mass and gas-phase met allicity. We derive gas-phase oxygen abundances and stellar masses using new techniques which make use of the latest stellar evolutionary synthesis and photoionization models. We find a tight ( ±0.1 dex) correlation between stellar mass and metallicity s panning over 3 orders of magnitude in stellar mass and a factor of 10 in metallicity. The relation is relatively steep from 108.5 1010.5 M⊙h 70, in good accord with known trends between luminosity and met allicity, but flattens above 10 10.5 M⊙. We use indirect estimates of the gas mass based on the H α luminosity to compare our data to predictions from simple closed box chemical evoluti n models. We show that metal loss is strongly anti-correlated with baryonic mass, with low mass dwarf gal axies being 5 times more metal-depleted than L ∗ galaxies at z ∼ 0.1. Evidence for metal depletion is not confined to dwarf galax ies, but is found in galaxies with masses as high as 10 10 M⊙. We interpret this as strong evidence both of the ubiquity of galactic winds and of their effectiveness in removing metals from galaxy potenti al wells. Subject headings: galaxies:abundances — galaxies: evolution — galaxies: fun damental parameters — galaxies: statistics