It has long been understood in theory that price-match guarantees can be anticompetitive, but to date, scant empirical evidence is available outside of some narrow markets. This paper broadens the scope of empirical analysis, studying a wide range of products sold on a national online market. Using an algorithm that extracts data from charts, I obtain a novel source of data from online price trackers. I examine prices of goods sold on Amazon before and after two big-box stores (Target and Best Buy) announced a guarantee to match Amazon's prices. Employing both difference-in-difference and regression-discontinuity approaches, I robustly estimate a positive causal effect of six percentage points. The effect was heterogeneous, with larger price increases for initially lower-priced items. My results support anticompetitive theories which predict price increases for Amazon, a firm that did not adopt the guarantee, and are consistent with plausible mechanisms for the heterogeneous impact.