Social movements occur not only because of political opportunities but also due to a perceived threat to citizens. Popular contention has remained an important mode of political participation in Hong Kong since 1997 when its sovereignty was handed over to China. Many influential collective actions in Hong Kong occurred when residents felt a threat had arisen from policies made by the city government or Beijing. By examining the Anti-Extradition-Bill movement in Hong Kong, this paper explores how threat triggers and sustains social movements. It finds that threat both facilitates the mobilization of social movements and sustains them. Threat strengthens solidarity among movement supporters because of their shared concerns and goals. It sustains a movement when government responses confirm participants' belief in the continual existence of the threat. The Anti-Extradition-Bill movement deepened the distrust between local residents and Beijing, resulting in the promulgation of the National Security Law by Beijing in May 2020.