<p>The MUSIC Model of Motivation specifies that students’ perceptions of the course motivational climate—eMpowerment, Usefulness, Success, Interest, and Caring (MUSIC)—predict their motivation, which then predicts their engagement in the course. Although many studies have demonstrated that students’ MUSIC perceptions predict their engagement (e.g., effort), no studies have examined whether course motivation serves as a mediator between MUSIC perceptions and engagement. To address this issue, we used multiple-group structural equation modeling to determine whether students’ motivation in a course mediates the relationship between students’ MUSIC perceptions and their course effort. We surveyed 918 students in three undergraduate STEM courses (an engineering course, an in-person mathematics course, and an online mathematics course) at a university in the US. In all three courses, students’ MUSIC perceptions significantly predicted their course motivation, which subsequently predicted their effort, and this relationship was invariant across courses. The five MUSIC constructs explained most of the variance in students’ course motivation, which in turn accounted for about a quarter of the variance in their effort. This study provides the first evidence to support the theoretical assertion that course motivation serves as a mediator between students’ MUSIC perceptions and engagement, as predicted by the MUSIC model theory.</p>