This study, derived from campaign communication mediation models, examines how incidental news exposure on social media affects political participation. Analysis of two-wave panel data collected before the 2016 US presidential election shows that incidental news exposure on social media is associated with increases in offline and online political participation (1) through online political information seeking and (2) through online political information seeking and online political expression in serial. Interestingly, results show that incidental news exposure on social media also has a direct negative relationship with offline and online political participation. Implications for the political utility of social media are discussed.