Purpose: The purpose of this study was to evaluate the limited efficacy of a high-intensity interval training (HIIT)-based fitness education unit in middle school physical education (PE). Method: The study took place in six PE classes at one middle school located in the southern United States. The authors conveniently assigned the classes to treatment (n = 3 classes; 113 students) or control (n = 3 classes; 119 students) groups. Two trained PE specialists implemented the HIIT lessons two to three times per week for 8 weeks. The authors collected mixed methods data at the student, class, and teacher levels for the evaluation. Results: The focus group teacher interview with the teachers, field observations, and accelerometer-determined in-class physical activity data revealed sound implementation fidelity. The HIIT-based fitness education condition also showed greater improvement in physical activity and fitness knowledge and attenuated decline in curl-up scores compared with the control. Conclusion: The findings support the limited efficacy of implementing HIIT for fitness education in middle school PE programs.