We use data on international migration to study the causal effect of gender discrimination on the sex-ratio of immigrants to the U.S. during the 1970-2019 period. We measure gender discrimination in the countries of origin using the Women, Business, and the Law (WBL) index, which measures legal differences in access to economic opportunities between men and women. Controlling for country fixed effects and regional time trends, as well as for potentially confounding factors, we find that a one standard deviation increase in the WBL index in a country of origin (a decrease in gender discrimination) decreases the share of women immigrating to the U.S. from that country by 1.7 percentage points, on average. This large effect of gender discrimination on the sex ratio of immigrants is robust to specification changes, and is not significant when examining senior citizens.