As a step towards the profitable employment of nanoparticles (NPs) in agriculture, effects of chitosan NPs was probed on barley plants under late season drought stress. A factorial experiment was performed based on a randomized complete block design with three replications. The experimental factors included the chitosan NPs concentrations (0 (control), 30, 60 and 90 ppm), application methods (foliar and soil application) and irrigation regimes (well-watered and withholding of irrigation for 15 days after pollination). The barley seeds were separately planted in pots. Then, the NPs were added to them through the soil and foliar application at three stages. The results indicated that using the chitosan NPs, especially 60 and 90 ppm, significantly increased the leaf area (LA), the leaf color (SPAD), the number of grain per spike, the grain yield and the harvest index compared to the control. Also, drought stress significantly decreased the yield and yield components compared to the well-watered plants. In contrast, using the chitosan NPs in plants under drought stress significantly increased the relative water content (RWC), the 1000-grain weight, the grain protein, the proline content, the catalase (CAT) and the superoxide dismutase (SOD) compared to the control. There was no a significant difference between two methods of using NPs in most studied traits. The results highlighted that using the chitosan NPs, especially 60 and 90 ppm, in both irrigation regimes can significantly improve the majority of the studied traits compared to the control and mitigate the harmful effects of drought stress.