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BACKGROUND Cotton (Gossypium hirsutum L.) is the fiber crop most widely cultivated globally and one of the most important commercial crops in China, irrigation is closely related to the growth of cotton. A water temperature for irrigation that is too low or too high inhibits cotton growth. Poor irrigation timing results in water and nutrient deficiencies that reduce cotton yield. Therefore, it is necessary to determine the appropriate irrigation timing and water temperature. METHOD We conducted an experiment in an arid region of north‐western China to assess the effects of irrigation timing and water temperature on soil temperature and the photosynthetic characteristics, biomass, total nitrogen (N), and seed cotton yield. Two irrigation times (daytime and nighttime) and four water temperatures (15, 20, 25, and 30 °C) were combined into eight treatments. RESULTS Our results showed that water warming and nighttime irrigation improved the photosynthesis, biomass, N concentration (the proportion of total N weight in the plant biomass, in g kg−1), N content (the mass of total N, in g plant−1), and cotton yield. The optimal water temperature range for photosynthesis was 25.7–28.7 °C. Water warming also boosted the biomass allocation to the stem and increased the N allocation to the stem and leaf. Nighttime irrigation enhanced these phenomena. Water warming also increased the number of bolls per plant but reduced the single boll weight, increasing the seed cotton yield by 5.88–11.46%. At the same water temperature, irrigation during the night increased the number of bolls per plant and the single boll weight, improving the seed cotton yield by 2.95–4.31%. Among them, NI25 (nighttime irrigation with 25 °C water temperature) increased the yield by 14.13–14.90% compared with CK (daytime irrigation with 15 °C water temperature), which offers the best combination for increasing the yield. CONCLUSION Our study clarifies the optimal irrigation timing and water temperature for cotton production under drip irrigation with film mulching, providing valuable information for improving the cotton yield in arid areas with temperate continental climate. © 2023 Society of Chemical Industry.