Background <p>Adolescence is a crucial stage for forming long-term health and nutritional habits. Many healthcare providers lack formal training in adolescent nutrition. This study assessed the effectiveness of a training course grounded in Merrill’s First Principles of Instruction in enhancing healthcare providers’ knowledge and satisfaction.</p> Methods <p>This quasi-experimental study employed a pretest-posttest control group design with three measurement points and was; conducted in the Damghan Health Network (August 2024). A total of 100 health care providers were selected through convenience sampling and assigned to two groups: an intervention group (<i>n</i> = 40) and a control group (<i>n</i> = 50). The data collection tools included a 12-item nutrition knowledge test and 10-item satisfaction questionnaire. The training for the intervention group was designed usingaccording to Merrill’s five principles, whileereas the control group received standard training. Data were analyzed using ANCOVA.</p> Results <p>The average knowledge score in the intervention group increased from 7.52 ± 1.92 to 10.71 ± 1.70 (42% increase), while the control group improved from 7.31 ± 1.47 to 9.17 ± 1.70 (25% increase). After adjusting for the pretest, the difference between groups was statistically significant (F = = 35.64, <i>p</i> &lt; 0.001, η²= = 0.28, d = = 0.9). At the two-week follow-up, the intervention group maintained a higher mean score (8.27 ± 2.41) compared to baseline (<i>p</i> = 0.02). Participant satisfaction was also significantly higher in the intervention group (42.3 ± 4.1 vs. 36.8 ± 5.2; <i>p</i> &lt; 0.001).</p> Conclusions <p>Training based on Merrill’s principles significantly improved providers’ knowledge and satisfaction regarding adolescent nutrition, supporting its use in health education programs.</p>