High temperature stress inhibits wheat photosynthetic processes and threatens wheat production. Photosynthetic heat tolerance (commonly measured as T_crit – the critical temperature at which incipient damage to photosystem II occurs) in wheat genotypes could be improved by exploiting genetic variation and genotype-by-environment interaction (GEI) for this trait. Flag leaf T_crit of a total of 54 wheat genotypes were evaluated in 12 thermal environments over three years in Australia using linear mixed models for assessing GEI effects. Nine of the 12 environments had significant genotypic effect and highly variable broad-sense heritability (H² ranged from 0.15 to 0.75). T_crit GEI was variable, with 55.6% of the genetic variance across environments accounted for by the factor analytic model. Mean daily growth temperature preceding anthesis was the most influential environmental driver of T_crit GEI, suggesting varied scales of biochemical, physiological, and structural adaptations to temperature requiring different durations to manifest at the thylakoid membrane and leaf levels. These changes help protect or repair photosystem II upon exposure to heat stress. To support current wheat breeding, we identified genotypes superior to commercial cultivars commonly grown by farmers, and showed that there is potential for developing genotypes with greater photosynthetic heat tolerance.