The growing need for sustainable viticultural strategies has led to increased interest in the use of biostimulants. In this study, we evaluated the effects of foliar application of an amino acid-based biostimulant (Naturamin®) at two doses (500 and 1,000 g/ha) on yield components and grape quality in Vitis vinifera L. cv. Tempranillo under rainfed conditions in La Rioja, Spain. While no significant effects on yield parameters were observed, the highest dose of Naturamin® significantly increased the concentration of several free amino acids in the grape must. These changes could enhance yeast nutrition and fermentation kinetics, while also contributing to the formation of aroma-active compounds and improving the sensory profile of wines. Additionally, a significant increase in hydroxycinnamic acid levels was detected, which may influence wine color stability and aromatic complexity. Interestingly, treated grapes reached phenolic maturity at lower sugar levels, which may be advantageous for producing balanced, lower-alcohol wines in the context of climate change. These results highlight the potential of amino acid-enriched biostimulants as a sustainable tool to improve grape composition without compromising yield. For the first time, we report a comprehensive evaluation of the effects of foliar application of the amino acid-based biostimulant Naturamin® on grapevines under rainfed conditions.