Currently, one of the main demands of consumers - especially in large fruits such as watermelon - is for supermarkets to use techniques of non-invasive analysis to enable them to measure the sweetness of the fruits at the time of purchase, and thus avoid having to base the choice exclusively on external appearance. In addition, increasing interest is being shown by consumers in knowing the nutritional quality of healthy foods, such as watermelon. Near infrared spectroscopy (NIRS) was used to assess in situ the physicochemical and nutritional quality of half-watermelons, which is the format in which they are usually sold in supermarkets at the beginning of the season, due to their high price. A handheld, new-generation spectrophotometer was used for this purpose, and two modes of analysis, static and dynamic, were studied. The results obtained show the viability of using NIRS technology in dynamic mode at the supermarket level to obtain a reliable measurement of the sweetness of the half-fruits, thus meeting the consumers’ demand for sweet-tasting fruits. Promising results were also obtained for measuring the antioxidant activity of the half-watermelons, thus paving the way for the nutritional labelling of this healthy food at the supermarket level. •NIRS technology to ensure consumers loyalty in future watermelon purchases.•Watermelons can label with nutritional information using NIRS technology.•NIRS inform consumers in situ of the degree of sweetness of the half-watermelons.•A portable new generation NIRS sensor was evaluated for in situ analysis.