Reductions in potato (Solanum tuberosum L.) canopy growth are observed with mild water stress. Potato growth is enhanced by elevated atmospheric carbon dioxide ([CO2]), but interactions of [CO2] and water stress on canopy formation and dry matter partitioning have not been studied. Two soil-plant-atmosphere research (SPAR) experiments were conducted at 370 or 740 µmol mol–1 [CO2] and six different irrigations from 10 to 100% of the daily water uptake of the control. Increases in plant length from 23 to 111 cm at 60 d after emergence (DAE), leaf appearance duration from 38 to 71 d, leaf appearance rate from 0.5 to 0.93 leaves d–1, individual leaf area from 50 to 175 cm2, and lateral branch elongation were observed as irrigation increased. Values were generally smaller for elevated [CO2] plants under water stress. Biomass increased with irrigation from 73 to 346 g plant–2. The percentage allocated to the canopy increased with irrigation from 50 to 80% in ambient and 30 to 80% in elevated [CO2]. Despite decreased canopy size, elevated [CO2] plants produced similar total biomass, but higher yield, at most irrigations. Reduced canopy mass in elevated [CO2] plants was attributed to suppressed lateral branch development due to an interactive effect of [CO2] and water stress on tuber sink strength. These results indicate that water stress predicted by climate change models will be mediated somewhat under [CO2] enrichment.