The need for justification is a widely observed social phenomenon. This paper develops a theoretical framework and reports laboratory evidence to show how pure justification pressure affects cooperative behavior in economic exchange environments. In a one-shot anonymous interaction, compared with the case when the behavior is simply observed by the audience, individuals are more likely to act on what they believe the audience thinks they should do when they also have to explain the decisions to the audience. When it is salient that the audience thinks one should cooperate, justification pressure significantly promotes cooperation even absent negative consequences for non-cooperative behavior. We discuss the implications of our findings for shaping institutional design to promote cooperation.