Understanding the relationship between stomatal conductance (g _s) and photosynthesis (A _n) under water stress conditions can improve the accuracy of land surface models for estimating the gas exchange of crop canopies with the atmosphere. However, little is known about the effect of water stress on this relationship in crops. A glasshouse experiment was, therefore, conducted to investigate changes in the relationship between g _s and A _n owing to water stress in rice and the association with soil moisture content. Severe (SWS), mild (MWS), and no water stress (NWS) conditions were applied from flowering onwards and the gas exchange in fully developed flag leaves was assessed weekly. The Ball-Woodrow-Berry linear model was used to assess the relationship between g _s and A _n under different treatments. SWS had a significant effect and reduced the slope ( a) of the relationship between g _s and A _n by 30% compared with NWS. Only in SWS were A _n and g _s strongly correlated with soil water content. Our study revealed that changes in the relationship through a reduction in a imply a conservative water-use strategy for rice under intense water stress. We propose that land surface models should consider water stress conditions when estimating CO ₂ and H ₂O fluxes from crop canopies.