Olive trees are among the best-adapted species to the Mediterranean basin and the Middle East, already characterized by persistent rain deficits. However, this area is considered a climate change hotspot for the increasing drought occurrence and warming predicted under future greenhouse gas emission scenarios. Indeed, drought stress is one of the major environmental constraints which detrimentally affects plant and fruit yields. However, it has been also broadly acknowledged the impressive effect of melatonin (MEL) treatment in mitigating the deleterious effects of this stress. In this view, our research was carried out to assess the potential advantageous effects of foliar MEL treatment exogenously (0 and 100 µM) on olive fruit quality, chemical profile, and yield of olive oil from Sevillana and Roughani olive cultivars exposed to various water stress levels (100, 75, and 50% ET). The data showed that independently of plant treatment, Sevillana oil exhibited more C16:1, C18:1, MUSFA/PUSFA, total chlorophylls, and carotenoids, while Roughani presented higher C16:0, C18:0, and phenolics. Olive oil from plants exposed to drought stress underwent several changes in fatty acid composition. For instance, under water stress, C16:0, C18:0, and C18:1 showed an increasing trend whereas the opposite was seen for C18:2 and C18:3 which declined. Therefore, while unsaturated fatty acid (USFA) significantly decreased, together with total chlorophylls, saturated fatty acids (SFA) considerably raised. Moreover, phenols and flavonoids, which are well-known to preserve membrane structure and prevent oxidative stress, increased under water deficit. MEL treatment, which has been proved promising in reducing membrane lipid peroxidation, further increased phenols, and carotenoids in the two cultivars under mild and strong water deficit, C16:1, C18:1, and C18:2 compared to respective controls. In conclusion, MEL-foliar treatment could cope with the deleterious drought effects by preservin
