Research suggests that optimism is positively related to emotional wellbeing. However, little is known about the emotion regulation mechanisms that potentially explain (i.e., mediate) this relationship. The present study was designed to address this issue. To this end, it examined emotion regulation strategies (i.e., appreciation, rumination, problem solving, and avoidance) that are theoretically and/or empirically related to optimism as possible mediators of the optimism-emotional wellbeing relationship. In the first of two phases, we established a baseline measure of dispositional optimism among 218 French adults. In the second phase, an experience sampling method was used to assess emotional wellbeing and emotion regulation strategy use over a 1-month period. A multiple mediation analysis suggested that the optimism-emotional wellbeing relationship is significantly mediated by appreciation and rumination but not by problem solving and avoidance. More specifically, engaging more in appreciation mediated 41% of this relationship, while engaging less in rumination mediated 20% of it. The results are discussed in light of the current knowledge on optimism, wellbeing, and emotion regulation.