We study how electoral systems affect the presence of women in politics in the context of Spanish municipal elections, in which national law mandates that municipalities follow one of two electoral systems: a closed-list system in which voters pick one party-list, or an open-list system, in which voters pick individual candidates. Using a regression discontinuity design, we find that the closed-list system increases the share of women among candidates, councilors, and mayors, by 4.1, 4.8, and 7.1 percentage points, respectively. We develop a model that allows us to test for three possible mechanisms: gender differences in the supply of candidates between the electoral systems, voter bias, and party bias. Model estimation indicates that a combination of supply differences and party bias best explains the results.