This study investigated the predictive validity of school-subject-specific self-concepts of ability, intrinsic task values, and cost (operationalized as task effort) for motivation regarding unclassified novel learning content--a fictional project management course--as a function of perceived similarity between school subject and novel learning content. Using an experimental survey design, perceived similarity was induced by presenting a description of the project management course that had some similarity to either mathematics or German (i.e., the school subject). Participants (N = 582) were randomly assigned to one of two experimental conditions (i.e., course description hinting at mathematical versus German-related content). Results from multiple-group structural equation modeling demonstrated that individuals' mathematics-specific self-concept of ability and intrinsic task value predicted their course-specific motivational beliefs only when the course description emphasized mathematical content. Facing mathematical course content, predictive validity of individuals' mathematics-specific self-concept of ability and intrinsic task value, respectively, was higher than their German-specific self-concept of ability and intrinsic task value. Findings regarding cost were less consistent. Results are discussed with regard to expectancy-value theory and task choice in higher and further education.