Purpose <p>Global warming, with rising temperatures and increasing drought conditions, poses a serious threat to agriculture, accelerating soil degradation, reducing crop yields and compromising production quality. In this work, we investigated the effects of different compost teas (CTs) on the growth, physiological and biochemical responses of lettuce under drought stress in a greenhouse experiment.</p> Methods <p>Four composting bioformulations were prepared by incorporating steel slag into horse manure to evaluate its impact on final compost teas properties. The windrows consisted of C1, C2, C3, and C4 with 0%, 10%, 25%, and 50% steel slag, respectively. Compost teas (CT1, CT2, CT3 and CT4) were extracted accordingly and diluted to 25%, 50%, 75%, and 100%. The compost teas were then applied to lettuce plants under drought stress in a greenhouse trial to evaluate their impacts on plant growth, quality, physiology, biochemical responses.</p> Results <p>CT2 and CT3 both at 50% dilution, significantly improved the biomass, physiological and biochemical parameters of lettuce under water-stressed conditions. In fact, CT2 enabled a 1.7-fold increase in dry biomass compared with control plants. Regarding biochemical performance, the application of CT2 and CT3, led to significant reductions in malondialdehyde content with significant reductions of 52% and 64%, respectively, compared to the control under stressed conditions. Additionally, these formulations were the most effective in enhancing total soluble protein content and antioxidant activity under both regimes.</p> Conclusion <p>The study identified 15 mL per pot per week of compost tea containing 10–25% steel slag at 50% dilution as the optimal application rate under greenhouse conditions to mitigate drought effects. This biostimulant exemplifies a circular-economy approach by valorizing industrial by-products such as steel slag for sustainable crop production.</p>