Background <p>A plant-based diet is reportedly effective in preventing chronic and metabolic diseases, primarily because of the antioxidant properties of phytochemicals present in vegetables and fruits. However, systematic metabolomics studies investigating the changes in metabolic phenotypes resulting from plant-based diets in adolescents are lacking. Therefore, this study aimed to examine alterations in urinary metabolomic profiles associated with a plant-based diet to explore its beneficial metabolic effects.</p> Methods <p>Eighteen healthy Korean adolescents participated in an 8-week intervention, in which they modified their dietary patterns to primarily include whole grains, vegetables, and fruits. Urine samples were analyzed using gas chromatography–mass spectrometry, and multivariate analysis was performed to identify statistically significant metabolite alterations following the plant-based dietary intervention.</p> Results <p>The plant-based diet significantly reduced body weight and body fat mass but increased plasma biological antioxidant potential. Metabolomic profiling revealed a clear distinction between samples collected before and after the 8-week intervention, with R²Y and Q² values for the permuted models at 0.906 and 0.635, respectively. Key metabolite changes included increases in L-phenylalanine, L-tyrosine, glutathione, succinic acid, and tartaric acid as well as decreases in L-leucine, glycine, and indoleacetic acid. These changes suggest enhancements of antioxidant capacity, detoxification pathways, and mitochondrial function.</p> Conclusion <p>This nutritional metabolomics study provides mechanistic insights into the metabolic changes associated with a plant-based diet. Its findings support the integration of plant-based meals into school settings to promote the metabolic health of adolescents and mitigate the risk of chronic disease progression.</p>