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Agriculture, a highly water-consuming sector, uses about 70 % of potable water for irrigation, which affects the availability of this scarce and limited resource. For decades, the biostimulant action of aqueous extracts of humic substances from cattle manure vermicompost has been demonstrated in numerous crops, and the protective effect against abiotic stress conditions has also been proven. Lettuce (Lactuca sativa L.) is a waterdemanding vegetable that responds to the stimulating effect of humic extracts. For these reasons, the aim of this work was to evaluate the effect of the application of a humic extract of vermicompost (HEV) on lettuce plants grown under low water supply. Experiments were conducted under controlled conditions, treating the plants with a combination of three dilutions of the obtained HEV and two water conditions (25 and 50 % of the maximum water holding capacity (MC), following a completely randomized experimental design. At 42 days after plant emergence, biological productivity indicators and biochemical-physiological indicators were evaluated. The humic extract of vermicompost presented advantageous physicochemical properties for use as a biostimulant. Plants sprayed with dilutions of the extract and under conditions of low water supply showed similar or higher values than the control with adequate water supply (MC 85 %). Increases in proline and leaf protein content were observed, but not in relative chlorophyll content. It is recommended to perform the total characterization of vermicompost extracts and to study the response of lettuce under field conditions. [ABSTRACT FROM AUTHOR]