Future emission scenarios can interfere with the yield of major crops. In this study, we investigated the future impact of increased air temperature and relative humidity on melon phenology and water demand in the Brazilian semi-arid region. We applied the PRECIS (Providing Regional Climates for Impact Studies) climatological model to develop the Intergovernmental Panel on Climate Change emission scenarios—B2 optimistic emission scenario and A2 pessimistic emission scenario—and we assessed the climate change effects on the phenology and water demand of two melon cultivars. The “Orange County” hybrid, the Honeydew melon, grew from 2006–2007, and the “Néctar” hybrid, the Galia melon, grew in 2008. These cultivars were also considered using the actual emission scenario. We found that the B2 and A2 emission scenarios will cause a cycle decrease of 15.49 and 25.35% for the “Orange County” hybrid and a 9.84 and 18.03% decrease for the “Néctar” hybrid. Future changes to the climate will increase the melon crop coefficient and daily rate of evapotranspiration. Regarding the “Orange County” hybrid, the cycle shortening overcomes the daily water demand increases, decreasing water demand by 13.7–18.3%. Regarding the “Néctar” hybrid, cycle shortening will be proportional to the increase in water demand. The Honeydew melon will be more sensitive to air temperature and relative humidity increases than the Galia melon.