There is increasing interest in reducing fertilizer cost and establishing proper irrigation management for sustainable vegetable production. Nigari, an effluent of salt industries, is cheaper than commercial fertilizers. Another important vegetable production factor is nutrient application timing to improve soilless cultivation in crops like sweet pepper. Therefore, this study evaluated the effects of nigari and nutrient solution application timing on leaf gas exchanges of sweet pepper cultivated under a soilless system. Treatments included three nigari rates [no nigari plus a standard nutrient solution as control, 2 mL·L−1 nigari + additional nitrogen–phosphorus–potassium (N–P–K) to equal the standard, and 4 mL·L−1 nigari + additional N–P–K to equal the standard]. Three daily application timings (T1 = 0700 hr + 1500 hr, T2 = 0900 hr + 1500 hr, and T3 = 0700 hr + 0900 hr + 1500 hr) were used for each nutrient solution. Leaf gas exchange parameters were studied during the vegetative and reproductive growth stages of sweet pepper cv. Papri new-E-red. Photosynthetic responses and its related parameters, namely transpiration (E), stomatal conductance (g S), and maximum photosynthesis (Amax), were significantly affected by nigari rates and nutrient solution application timing. Photosynthesis-related parameters, E, g S, Amax, and initial slope of photosynthesis in response to light curve were the highest and light compensation point (LCP) and leaf vapor pressure deficit (LVPD) were the lowest at 2 mL·L−1 nigari compared with the control at both plant vegetative and reproductive growth stages. For nutrient solution application timing, the highest E, g S, and Amax were observed at T3 treatment at both plant growth stages. Furthermore, marketable yield of sweet pepper was the highest when 2 mL·L−1 nigari was applied at 0700, 0900, and 1500 hr a day. Leaf gas exchange parameters showed that nutrient solution application timing of 0700, 0900, and 1500 hr a day was better for obtaining high yield of sweet pepper under nigari treatment in soilless culture.