The Supplemental Nutrition Assistance Program (SNAP), which helps millions of poor and low-income Americans purchase food, is the nation’s largest nutrition assistance program. This analysis estimates SNAP’s effect on poverty using the Supplemental Poverty Measure (SPM). We augment the Census Bureau’s SPM to correct for the underreporting of SNAP and other means-tested benefits in the underlying survey data. We find that SNAP removed 8.4 million people from poverty in 2015, reducing the poverty rate from 15.4 percent to 12.8 percent (a reduction of 17 percent). SNAP reduced the poverty gap (the aggregate amount of additional income required to remove all poor families from poverty) by $35 billion (21 percent) in 2015.