AbstractWater availability is one of the main factors affecting crop production and the occurrence of drought periods is expected to increase in the context of ongoing climate change. We investigated the impact of water stress on two pseudocereal species, common buckwheat (Fagopyrum esculentum) and Tartary buckwheat (Fagopyrum tataricum). Plants were grown under greenhouse conditions under two water regimes: control (40–50% soil humidity) and water stress (<20% soil humidity). Although closely related, both species differed by their resistance to water stress. The vegetative growth was affected inF. esculentumbut not inF. tataricumas water stress decreased leaf production, leaf fresh, and dry weight, stomatal conductance, transpiration rate, and photosynthesis rate in the former but not in the latter. However, chlorophyll fluorescence parameters were not affected by water stress, whatever the species, and the chlorophyll content increased in water‐stressed plants in both species. Oxidative stress was observed in both species in response to water stress, and antioxidant content was increased inF. tataricum. The reproductive phase was affected by water stress in both species: the number of inflorescences and pollen production decreased, mainly inF. esculentum. Seed set was maintained inF. tataricumwhile this parameter was not investigated inF. esculentumdue to its self‐incompatibility. Our results suggested thatF. tataricumwas more resistant to water stress thanF. esculentumand thatF. esculentumhad characteristics of drought avoidance, whileF. tataricumexhibited traits of drought tolerance.