How to support young people's agency is a key question of sustainability education. Action competence for sustainability is suggested covering the essential components of youth's readiness to act both individually and collectively, but empirical tests of these associations are scarce. Therefore, this study explores the relationship between action competence and sustainability action among young people. We conducted a national survey of 15 to 29-year-old youths (N = 940) in Finland. Results of structural equation modeling show that self-perceived action competence for sustainability predicts private sphere behavior, but not sustainability action. Further analysis on the components of action competence reveals that high perceived knowledge and low outcome expectations predict low engagement in sustainability action. Knowledge and outcome expectations also affect behavior indirectly via willingness to act. We argue that actions and behaviors have different antecedents, and that the ability to recognize outcome uncertainty affects how young people's sustainability agency is manifested.