Grafting on disease-resistant rootstocks is a growing practice in watermelon cultivation worldwide. Reports on effects of grafting on watermelon fruit postharvest performance are scarce. The current work examined postharvest performance at 25C of four diploid cultivars grown nongrafted or grafted onto three Cucurbita maxima × C. moschata rootstocks. Despite sucrose accumulation throughout storage at the expense of fructose and glucose, total soluble carbohydrates and soluble solids content declined. Lycopene content peaked 7 days postharvest and intensity of flesh color increased concomitantly; yellowing of flesh was detected at 14 days. Rootstocks reduced soluble solids content only in the second year of experiments by 0.8–1.0°Brix. Rootstocks improved postharvest flesh firmness and lycopene content and enhanced flesh color. Rind was minimally thickened by rootstocks and declined with storage. Grafting diploid cultivars on interspecific rootstocks improved fruit quality and storability. Potential reduction in soluble solids content in response to grafting is limited and not detrimental to fruit quality.