Aluminum stress affects the growth and photosynthesis of watermelon seedlings. One variety was more sensitive to aluminum than the other. Chlorophyll content increased in one variety under aluminum treatment. Stomatal conductance and photosynthetic rates were more affected by aluminum in one variety at lower concentrations. Chlorophyll fluorescence parameters decreased, while non-photochemical quenching increased under aluminum stress. The ability of leaves to transform energy decreased, but they maintained a high energy distribution. Overall, the electron transport rate was less affected by increasing aluminum concentration.