| Summary: | γ-Aminobutyric acid (GABA) occurs naturally at a low concentration in fruits, but can be increased following several stress events, playing a physiological effect. Lemon trees were preharvest treated with GABA at three concentrations (10, 50, and 100 mM) during two consecutive seasons (2019–2020 and 2020–2021). Fruit growth (diameter) and crop yield (kg tree<sup>−1</sup> and number of fruits tree<sup>−1</sup>) and quality traits were evaluated at harvest. Results showed that treatments were effective at increasing lemon size (a 5% higher) and yield, especially for GABA at 100 mM, for the two assayed seasons. Thus, yield was increased between 13 and 18% with respect to the control trees for the two harvest dates. With respect to the quality traits, GABA treatments did not impact any negative effects on the quality attributes, since the total soluble solids (7–8° Brix), total acidity (5–6 g 100 g<sup>−1</sup>), and fruit firmness (13–14 N mm<sup>−1</sup>) were similar to the control fruits. Therefore, GABA applied as preharvest treatment could be considered as a potent tool to enhance the yield of lemon fruits.
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