Third-party diplomatic support for protest movements has become an increasingly important tool for international actors in order to inform and influence foreign publics, especially, discontented citizens. Expressing official support for protest campaigns -through condemning government repression, encouraging political reform, praising protesters, and sympathizing with them- or imposing economic sanctions because of the host government's repressive behavior is now a significant component of diplomacy. Despite the growing importance of third-party diplomatic support for protest movements, little systematic research has been conducted on its causes and consequence. This study includes three interrelated papers to address this gap in the literature. The first paper addresses the question of why countries provide diplomatic support for protest movements. Focusing on Western diplomacy in the post-Cold War era, I argue that there are two reasons why Western democracies take diplomatic actions in support of protest campaigns. First, when the host government uses extreme violence against protesters; and second when the host government has an oppositional stance toward the U.S.-led liberal international order. I use original data of 523 diplomatic actions from 1990 to 2019 to test these theoretical expectations. The second paper asks whether and under what conditions diplomatic support is effective. I argue that diplomatic support is likely to reduce popular support or protest movements when it occurs alongside protesters' call for international help because it makes governments' common claim on the alleged coordination between opposition figures and foreign countries credible. I conduct a survey experiment among Iranian intelligentsia to test this argument. The results support the theoretical model. The third chapter asks whether the identity of supporter could make a difference in the public's attitudes toward protest movements. The central argument is that support from strategic allies is likely to weaken public support for protest campaigns. However, when diplomatic support comes from allies, it is likely to increase support for protesters. I test this theory by conducting a survey experiment of 1800 American citizens on the Black Lives Matter Movement. The findings show that the identity of supporters plays a crucial role in shaping public attitudes toward protest movements.