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Water is critical for plant growth. To improve the water retention of soil, we synthesized a superabsorbent hydrogel (Gel-1) derived from the food waste okara as a soil supplement. We showed that Gel-1 amendment significantly improved growth (shoot fresh weight, digital biomass, leaf area, relative growth rate) and irrigation water-use efficiency (IrrWUE) of Asian leafy vegetable Brassica rapa. The yield was increased by 88% and 113% when the vegetables were grown in substrate supplemented with 3 and 5% (w/w) of Gel-1, respectively. IrrWUE was significantly compromised when watering frequency was increased from two to four times in two weeks to prevent wilting in the absence of Gel-1. Increasing watering frequency to >5 times in two weeks without Gel-1 amendment led to >98% nitrogen loss, adversely affecting plant growth significantly. Restoring nitrogen using Hoagland's solution improved plant growth confirmed frequent irrigation resulted in nutrient leaching, affecting plant performance. In contrast, incorporation of Gel-1 reduced leaching and significantly improved IrrWUE (>80%), under water-limited conditions. Our findings suggest Gel-1 could reduce half of the watering frequency, save ~50% of water and prevent excessive nutrient leaching, thereby enhancing growth. Potential applications of such okara-based hydrogel for agricultural purposes are discussed.