Can engaging college students in client-centered projects in science, technology, engineering, and mathematics (STEM) coursework increase interest in STEM professions? The current study explored the effectiveness of project-based learning (PjBL) courses on student attitudes, major choice, and career aspirations in STEM. Framed in expectancy-value and social cognitive career choice models, we examined the effect of engaging in at least one authentic, project-based course during the first four semesters of college on student STEM attitudes and career aspirations in a quasi-experimental study with a sample of (N = 492) natural science and engineering students. STEM self-efficacy and subjective task value variables (STEM attainment, intrinsic and utility value of STEM courses, and relative cost associated with engaging in STEM courses) were examined as mediators of the relationship between classroom project-based experiences and STEM career aspirations. Gender and underrepresented minority status were also examined. We found that engaging in at least one project-based course during the first four semesters affected student perceptions of STEM skills, perceptions of the utility value of participating in STEM courses, and STEM career aspirations. Furthermore, we found that the effect of project-based courses on STEM career aspirations was mediated by STEM skills and perceptions of course utility. The effect of PjBL was not moderated by race or gender. We highlight areas of future research and the promise of PjBL for engaging students in STEM professions.