The relationship between social media use and youth’s political participation has been extensively studied. However, explanations for youth’s online collective political activism have been less explored. Previous studies have used the concept of internal political efficacy to examine the relationship between social media and political participation. However, this concept only explains individual political participation, while many political actions are performed collectively. Based on Social Cognitive theory this study propounds the concepts of online political self- and collective efficacy and explores their relationship to online collective political activism. Findings of a survey of members of three activist groups of a US Mid-Western university (n = 222) suggest that a correspondence exists between efficacy perceptions and the level of agency at which the political activities are performed online. Also, online collective efficacy perceptions influence individuals’ participation in online collective actions, but this relationship is moderated by the perceived interdependence of the actions.