• The interactive effects of atmospheric CO2 concentration and soil-water content on grain sorghum (Sorghum bicolor) are reported here. • Sorghum plants were exposed to ambient (control) and free-air CO2 enrichment (FACE; ambient+200 μ mol mol-1), under ample (wet, 100% replacement of evapotranspiration) and reduced (dry, postplanting and mid-season irrigations) water supply over two growing seasons. • FACE reduced seasonal average stomatal conductance (g s) by 0.17 mol ( H2 O) m-2 s-1 (32% and 37% for dry and wet, respectively) compared with control; this was similar to the difference between dry and wet treatments. FACE increased net assimilation rate (A) by 4.77 μmol ( CO2) m-2 s-1 (23% and 9% for dry and wet, respectively), whereas dry decreased A by 10.50 μmol ( CO2) m-2 s-1 (26%) compared with wet. Total plant water potential (Ψ w) was 0.16 MPa (9%) and 0.04 MPa (3%) less negative in FACE than in the control treatment for dry and wet, respectively. Under dry, FACE stimulated final shoot biomass by 15%. • By ameliorating the adverse effects of drought, elevated atmospheric CO2 improved plant water status, which indirectly caused an increase in carbon gain.