Do parties in Spain use Instagram and for what purpose? What characterizes posts that are effective at engaging the electorate? This article examines the use of Instagram during the Spanish general elections of 2015 and 2016. Content analysis of party publications on Instagram was carried out to test whether parties use the application to broadcast their policy positions, mobilize voters, or to promote their main party candidate. Regression estimation models are then used to explain the varying levels of user engagement with party posts. The findings illustrate that the new challenger parties, Podemos and Ciudadanos, use the social media app in a systematic way that is distinct from the traditional mainstream parties in Spain. Moreover, party newness, attempts at mobilization, and the promotion of the main party candidate are found to be important explanations for party success on Instagram.