Research on intergroup emotions has largely focused on the experience of emotions and surprisingly little attention has been given to the expression of emotions. Drawing on the social-functional approach to emotions, we argue that in the context of intergroup conflicts, outgroup members’ expression of disappointment with one’s ingroup induces the complementary emotion of collective guilt and correspondingly a collective action protesting ingroup actions against the outgroup. In Study 1 conducted immediately after the 2014 Gaza war, Jewish-Israeli participants received information about (...) outgroup’s expression of emotions. As predicted, outgroup’s expression of disappointment increased collective guilt and willingness to participate in collective action, but only among those who saw the intergroup situation as illegitimate. Moreover, collective guilt mediated the relationship between disappointment expression and collective action, moderated, again, by legitimacy perception. In Study 2, we replicated these results in the context of racial tension between Black and White Americans in the US. We discuss the theoretical and applied implications of the findings. (shrink)