In-app advertising is one of the fastest growing areas in social commerce. Building on previous studies of e-commerce and psychological theories, this paper examines a theoretical model that extends the theory of planned behavior to include the propensity to trust and trust as antecedents of mobile users' attitudes toward in-app advertisements. The model was tested with 480 young Chinese mobile users. Results of structural equation modeling indicated that users' propensity to trust affected their trust in in-app advertising, and this in turn affected their attitudes toward in-app advertisements and their intention to watch in-app advertisements. In addition, subjective norm and perceived behavioral control were found to positively predict users' intention to watch in-app advertisements, which in turn affected their behavioral response. Practical implications are provided to increase users' trust and promote favorable attitudes toward in-app advertisements.