Leaf carbon isotope discrimination (CID ) has been suggested as an indirect tool for breeding for water‐use efficiency (WUE ) in various crops. This work focused on assessing phenotypic correlations between WUE and leaf CID and analysing genotypic variability in four sunflower genotypes grown in a greenhouse in pots with five different stable levels of soil water content (SWC ). We measured WUE at whole plant and leaf (intrinsic) level. At whole plant level, WUE was derived from the ratio of total dry aerial biomass (BM ) to cumulative water transpired (CWT ). At leaf level, intrinsic WUE was calculated as the ratio of light‐saturated CO 2 assimilation to stomatal conductance (A /gs) in younger expanded leaves. Significant differences among the four genotypes and the five SWC s were observed for whole plant and leaf WUE and CID . Strong negative correlations were observed between whole plant WUE and CID as well as between intrinsic WUE and CID with decreasing water availability. No relationships appeared between BM production and WUE or CID . Our results can help agronomists and breeders to evaluate sunflower lines with high WUE for adaptation to drought conditions and for reducing water consumption and crop water needs. Leaf CID appears to be a pertinent and valuable trait to select sunflower genotypes with high WUE .