I measure the effects of street-level political advertising on voting behavior, estimating not only how parties’ ads affect their own vote shares but also other parties’ vote shares. I use a novel dataset on ad location in a major Spanish city during elections for the national parliament as well as granular socio-economic and voting data. This set-up, where more than two parties are running for office and elections are very competitive, allows me to explore the heterogeneous effects of ads across parties as well as how parties’ ads affect other parties’ vote shares. To identify the effects of parties’ ads, I exploit legally mandated randomized assignment of ad location to parties across multiple years. I find that a party’s own ads have a positive effect on a party’s vote share, although the effects are quite heterogeneous across parties. A one standard deviation increase in the number of ads increases a party’s vote share by 0.79 percentage points on average. Ads of parties that have a platform that is ideologically distant always have a negative effect on a party’s vote share. Instead, ads of parties that are close competitors may act either as complements or substitutes in different years.