Adolescents in Nairobi, Kenya face nutritional challenges that can impact their health outcomes. A cross-sectional study was conducted among adolescents participating in a nutrition education programme at four schools in Nairobi. Diet quality scores were assessed using the Healthy Eating Index (HEI), and health outcomes such as BMI and blood pressure were measured. Adolescents who participated in school-based nutrition education had significantly higher HEI scores (mean difference of 5 points, $t(304) = 2.87$, $p < 0.01$; 95% confidence interval: 1.2 to 8.6). School-based nutrition education programmes appear effective in improving adolescent diet quality. Further longitudinal studies are recommended to assess long-term health benefits. Adolescents, Nutrition Education, Diet Quality, Health Outcomes, Nairobi, Kenya