Climate models predict greater increases in nighttime temperature in future climates. The impact of high nighttime temperature on wheat is not well understood. The objectives of this research were to investigate the influence of high nighttime temperatures during reproductive development on phenology, physiological, vegetative, and yield traits of wheat. Two spring wheat cultivars (Pavon and Seri-82) were grown in controlled environment chambers at optimum temperatures (day/night, 24/14°C; 16/8 h photo/dark period) from sowing to booting and thereafter plants were exposed to four different nighttime temperatures (14, 17, 20, 23°C) until maturity. The daytime temperature was similar at 24°C across all treatments. Data on phenology (time to panicle emergence, flowering and maturity), physiological traits (photosynthesis, stomatal conductance, transpiration, chlorophyll fluorescence and leaf chlorophyll content) were measured at frequent intervals. At maturity, numbers of tillers and ears, shoot dry weight, spikelet fertility (proportion of filled to total grains per ear), seed-size and seed yield were determined. There was significant influence of high night temperatures (>17°C) on growth and yield traits, but not cultivar by temperature interactions. High night temperature did not influence photosynthesis until 14 d, but was decreased at 21 d after stress. Grain filling duration was decreased by 3 and 7 d, respectively, at high night temperatures of 20 and 23°C, when compared to 14°C. High night temperature (23°C) decreased spikelet fertility, while night temperature >20°C also decreased the seed-size. High night temperatures (20 and 23°C) led to decreases in grain yield and harvest index in both cultivars.