We study the effects of municipal mergers on voter turnout in a difference-in-differences framework, using data from a wave of municipal mergers in Finland in 2009. Analysing two pre-merger elections and three post-merger elections, spanning a total of 17 years, we find that municipal mergers decrease voter turnout by 4 percentage points in the long run in the relatively small municipalities compared to similar small municipalities that did not merge. As the average turnout rate prior to merging in this group was around 69%, this is a substantial effect. We also find that virtually nothing happens to turnout in the municipalities that were relatively large within their merger. Furthermore, mergers are associated with a decrease in voters’ political efficacy and turnout decreases more in those municipalities that experience larger decreases in efficacy.