Armies, churches, organizations, and communities often engage in activities-for example, marching, singing, and dancing-that lead group members to act in synchrony with each other. Anthropologists and sociologists have speculated that rituals involving synchronous activity may produce positive emotions that weaken the psychological boundaries between the self and the group. This article explores whether synchronous activity may serve as a partial solution to the free-rider problem facing groups that need to motivate their members to contribute toward the collective good. Across three experiments, people acting in synchrony with others cooperated more in subsequent group economic exercises, even in situations requiring personal sacrifice. Our results also showed that positive emotions need not be generated for synchrony to foster cooperation. In total, the results suggest that acting in synchrony with others can increase cooperation by strengthening social attachment among group members.