In semiarid and arid climates in the Middle East, environmental stresses such as drought and water deficit result in significant reductions in the growth and productivity of horticultural and agronomic crops. To improve crop tolerance to water-deficient conditions, applying chitosan can be a practical approach. The foliar-spraying of medium molecular weight chitosan (450 kDa, 95-98% degree of deacetylation, and 30 mPa.s viscosity) under different irrigation regimes on the yield and phytochemicals of the Iranian landrace red pepper were studied. Experimental factors were irrigation system included drip and flood irrigation; irrigation frequencies included optimal irrigation (irrigation every 5 to 7 days based on irrigation at 85-90% field capacity or F.C.), deficit irrigation or 50% optimum irrigation (irrigation every 13 to 15 days based on irrigation at 45-50% F.C.); the foliar applications included negative control (no spraying), positive control (the foliar spraying by water as solvent), and foliar spraying by chitosan at 2 and 4 g L-1. The highest yields of the fruit were obtained from the pepper plants treated with foliar-sprayed chitosan under a drip irrigation system and optimal irrigation conditions. However, the maximum values of capsaicin (11.49 mg g-1 DW), dihydrocapsaicin (4.99 mg g-1 DW), capsaicinoids (16.48 mg g-1 DW), vitamin C (1.26 mg g-1 DW), total phenolic content (2.15 mg GAE 100 g-1 DW), and antioxidant capacity (55%) were achieved in the plants sprayed by chitosan at 2 g L-1 under 50% optimum irrigation (deficit irrigation) and drip irrigation system. The use of chitosan under water-deficit conditions (471 g capsaicin m-2) resulted in the highest capsaicin yield. In conclusion, chitosan foliar application under deficit irrigation is recommended to maintain and stabilize red pepper's quantitative and qualitative performance.