. Sustainable strategies to mitigate water stress and enhance fruit quality are therefore of immense industrial interest. This study investigated the efficacy of foliar potassium (K) application as a practical agricultural strategy to ameliorate the adverse effects of different irrigation levels on grape productivity and, more importantly, on key quality attributes relevant to the food processing industry. A field experiment was conducted using a factorial arrangement based on a Randomized Complete Block Design (RCBD). Grapevines were subjected to three irrigation levels (100%, 75%, and 50% of crop evapotranspiration - ETc) and three concentrations of foliar potassium (0, 1%, and 2% K2O). Parameters measured included yield, cluster weight, and a comprehensive set of quality traits: total soluble solids (TSS), titratable acidity (TA), pH, berry firmness, and the concentration of anthocyanins and total phenolics. Water deficit significantly reduced yield but enhanced most quality parameters. The 50% ETc treatment led to a notable increase in TSS, TSS/TA ratio, and anthocyanin content. Foliar potassium application, particularly at 2%, effectively mitigated yield losses under stress and synergistically improved quality. The interaction between 75% ETc and 2% K spray resulted in an optimal balance, achieving a yield comparable to full irrigation while significantly boosting TSS, phenolic content, and color intensity—attributes highly sought after for premium wine and juice production. The integrated management of deficit irrigation (75% ETc) and foliar potassium nutrition (2%) presents a viable and sustainable strategy for the grape industry.