Abstract: Women who visibly engage in politics online experience a lot of backlash. The presented study investigates sexist incivility against women in online political spaces as a possible explanation for the gender gap in online political discussion and expression. Online sexism solidifies the masculine norm in online political spaces. Drawing on social cognitive theory and the theory of normative social behavior, we understand online incivility as communication mechanisms that enforce gender norms in online political discussions. We use a preregistered online survey experiment with German Internet users to investigate how sexist comments in online political discussions affect women’s participation in the discussion, perceived social norms about participating, and their internal political efficacy. We found no effects of sexist comments on the propensity to engage in the discussion or the political efficacy to contribute. However, the presence of sexist comments increased the fear of sanctions in both men and women. The null findings of the preregistered experiment occurred despite sufficient statistical power and a successful treatment check. We discuss several possible explanations for the null effects and ways forward.