We assess whether and how accidental exposure to political information on social media contributes to citizens’ online political participation in comparative perspective. Based on three online surveys of samples representative of German, Italian, and British internet users in the aftermath of the 2014 European Parliament elections, we find that accidental exposure to political information on social media is positively and significantly correlated to online participation in all three countries, particularly so in Germany where overall levels of participation were lower. We also find that interest in politics moderates this relationship, so that the correlation is stronger among the less interested than among the highly interested. These findings suggest that inadvertent encounters with political content on social media are likely to reduce the gap in online engagement between citizens with high and low interest in politics, potentially broadening the range of voices that make themselves heard.