<p>Conventional Zn fertilisers have poor use efficiency (&lt; 5%), whereas the high reactivity and bioavailability of nano zinc oxide (ZnO NPs) offer a potential and efficient Zn carrier. With this background, the present study compared the efficacy of ZnO NPs with bulk ZnO and zinc sulphate (ZnSO₄) on improving seed germination, crop performance, and Zn fortification under both in vitro and field conditions. In an in vitro experiment, rice seeds were treated with bulk ZnO (500, 1000, and 1500 mg L^− 1) and ZnO NPs (50, 100, and 150 mg L^− 1) under variable soaking durations (24 and 48&#xa0;h), with an untreated control. ZnO NPs @ 150 mg L^− 1 treatment significantly enhanced germination percentage and seedling vigour compared to bulk ZnO. Based on these results, a field experiment was conducted to validate the performance of ZnO NPs under field conditions. Treatments included a control, bulk ZnO (500, 1000, and 1500 mg L^− 1), ZnO NPs (150, 250, 350, and 500 mg L^− 1), and ZnSO₄ (0.5%), applied as foliar sprays at flowering and grain-filling stages. ZnO NPs at 150 and 250 ppm increased grain yield by 15% and 21%, respectively, while ZnSO₄ increased yield by 27% over the control. Nano ZnO also enhanced chlorophyll <i>a</i> and <i>b</i> contents by 26–57% and 37–50%, respectively. Root length increased substantially at the optimal ZnO NPs (150 ppm) concentration but declined slightly at higher concentrations. Grain Zn content improved by increased by 1.53 fold with 150 ppm ZnO NPs, accompanied by higher physiological efficiency and Zn harvest index. Though ZnSO₄ resulted a relatively higher yield increase, foliar application of ZnO NPs demonstrated improved zinc accumulation and use efficiency at lower application rates, indicating their potential as an efficient alternative strategy for zinc nutrition in rice under Zn-deficient soils.</p>