Avocado (Persea americana Mill.) is a subtropical tree, particularly sensitive to either an excess or a lack of water. Understanding this balance is crucial to determine the optimum water supply and enhance crop productivity. The rising shortage of water resources in semiarid producing regions and the need for irrigation optimization call for sustainable water savings. A 3-year monitoring study with avocado cv. “Hass” tested sustained-deficit irrigation strategies supplying 33, 50 or 75% of the estimated crop water demand. They were compared with a control strategy supplying 100% of the estimated crop water demand. The leaf water potential, stomatal conductance, tree growth, yield and several fruit quality parameters were evaluated. The yield, tree growth and fruit size were significantly and proportionally lessened by deficit irrigation treatments. Over the study, the average yields amounted to 31.6, 38.3 and 43.8 kg tree−1 in the 33, 50 and 75% treatments respectively, which were significantly less than the yield of 47.5 kg tree−1 measured in the control. The 33% and 75% sustained-deficit irrigation treatments increased the omega 3 and omega 6 fatty acids as well as the unsaturated fatty acids (oleic); however, the 33% deficit irrigation treatment significantly reduced the yield, size and fruit weight. We consider, therefore, that approximately 25% of the irrigation water can be saved without adversely affecting the avocado tree performance in short and medium terms and we recommend implementing the 75% sustained-deficit irrigation strategy. Here we show for the first time the key role of redesigning irrigation strategies in Mediterranean areas, focusing on the benefits of sustained-deficit irrigation, which can save water, encourage water use efficiency and enhance fruit quality. In order to save water, it is critical and necessary to implement such novel water shortage strategies, at the affordable cost of reducing fruit yield while enhancing its quality.