Relevance interventions have shown a great potential to foster motivation and achievement (Lazowski & Hulleman, 2016). Yet, further research is warranted to test how such interventions can be successfully implemented in practice. We conducted a cluster-randomized trial in ninth-grade math classrooms to test the effectiveness of a relevance intervention, which was shown to be efficacious when implemented by researchers, for fostering motivation and achievement under real-world conditions. The 78 participating classrooms (N = 1,744 students) were randomly assigned to one of two intervention conditions or a waitlist control condition. The intervention was implemented by master's students or the regular math teachers. Intervention effects were evaluated using self-reports, teacher ratings, and achievement tests 4 weeks and 3 months after the intervention, controlling for the initial levels of the outcomes. Compared with the control condition, both intervention conditions showed similar positive effects on utility value. Unexpectedly, students in both intervention conditions also reported higher perceived cost compared with students in the control condition after the intervention. When implemented by master's students, additional intervention effects on students' growth mindsets and a standardized achievement test could be observed. Only small differences in effectiveness were observed between the intervention conditions, although master's students showed a higher level of adherence. In both intervention conditions, higher levels of adherence and lower levels of discipline problems were associated with more positive changes in utility value. Overall, the intervention thus showed mixed effects. Future research should therefore continue to examine the conditions under which relevance interventions work in practice.