Heat stress has always been a major determinant of mandarin production in Upper Egypt. To reduce the negative effects of such climatic changes on the mandarin trees in Assiut, proline was used to make the trees able to withstand these changes. Herein, the experiments were executed throughout two successive seasons (2019/2020 and 2020/2021) at the experimental orchard of Assiut University, Faculty of Agriculture, Egypt, on twelve uniform Balady mandarin trees in a complete randomized-block design (3 treatments x 4 replications). The main objective of this study was to enhance the fruit quality of the Balady mandarin cultivar by using different concentrations of L-proline (100 and 200 µg/ml) under Assiut conditions. Balady mandarin trees were exposed to L-proline five times through the fruit-growing period (every 30 days from June 1st week to October 1st week). The results demonstrated that five applications of 200 µg/ml L-proline foliar treatment might significantly boost Balady mandarin quality. Consequently, using this eco-friendly fertilizer under the climatic conditions of Assiut is recommended.