Objectives. To assess the effect of a 2017 excise tax on sugar and artificially sweetened beverages in Philadelphia, Pennsylvania, on the shopping patterns of low-income populations using Supplemental Nutrition Assistance Program (SNAP) data. Methods. I used a synthetic controls approach to estimate the effect of the tax on Philadelphia and neighboring Pennsylvania counties (Bucks, Delaware, and Montgomery) as measured by total SNAP sales ("SNAP redemption") and SNAP redemption per SNAP participant. I assembled biannual data (2005-2019) from all US counties for SNAP redemption and relevant predictors. I performed placebo tests to estimate statistically significant effects and conducted robustness checks. Results. Detectable increases in SNAP spending occurred in all 3 Philadelphia neighboring counties. Per-participant SNAP spending increased in 2 of the neighboring counties and decreased in Philadelphia. These effects were robust across multiple specifications and placebo tests. Conclusions. The tax contributed to increased SNAP shopping in Philadelphia's neighboring counties across both outcome measures, and decreased spending in Philadelphia (at least by 1 measure). This raises questions about retailer behavior, the effectiveness of the tax's public health aim of reducing sugar-sweetened beverage consumption, and policy aims of investing in low-income communities. (Am J Public Health. 2021;111(11):1986-1996. https://doi.org/10.2105/AJPH.2021.306464).