Viticulture is an essential sector in agriculture as wine production plays a vital role in the socio-economic life of many countries, especially in the Mediterranean area. Grapevines are a valuable, long-lived species able to grow in hot and dry regions. We currently do not know whether rain-fed grapevines entirely rely on deep soil water or make substantial use of shallow water from summer precipitation events. Without knowing this, we poorly understand what fraction of summer precipitation inputs contributes to grapevine transpiration. This has implications for how we quantify grapevine-relevant precipitation budgets and for predicting the impacts of climate change on grape and wine production. We investigated grapevine water use in a vineyard in the Chianti region, central Italy. During the growing season of 2021, we monitored precipitation and soil moisture at 30- and 60-cm depth. We collected over 250 samples for stable isotope analysis from rainfall, soil, and plants. Since traditional plant water sampling is problematic for grapevines, we collected samples from shoots, leaves, and condensed leaf transpiration after sealed plastic bags were wrapped around a shoot. We use these alternative plant samples to reconstruct the isotopic signal in the xylem water and infer the plant's seasonal water origin throughout the growing season. The analysis of the seasonal origin of water revealed that, throughout the growing season, soil water and plant water received disproportional contributions by rain that had fallen in the winter, even when compensating for the Mediterranean climate of the area. Only in late summer did the grapevines use substantial amounts of summer rainfall, whose contribution occasionally became dominant. These results provide a better understanding of ecohydrological interactions and uptake dynamics in valuable socio-economic agroecosystems such as vineyards.