Potassium (K) and phosphorus (P) are essential macronutrients for potato (Solanum tuberosum L.); they play crucial roles in photoassimilate production that can also be affected by elevated atmospheric CO2 concentration (e[CO2]). However, the interactive effects of K and P nutrition under elevated CO2 in potato has not been investigated. A pot experiment was carried out on potato plants in 1-L pots with five K supply rates at low P (LP) and high P (HP) in controlled environment chambers with ambient CO2 concentrations (a[CO2]) and an e[CO2] level of double a[CO2]. There was a significant interaction between K supply and P supply, but not between CO2 and K supply on total plant biomass, water use as well as water-use efficiency (WUE). K supply could remarkably enhance the accumulation of plant biomass under e[CO2] by promoting tuber formation. The maximum total plant biomass increased by approximately 1.3-fold under e[CO2] in this study, while the effect of CO2 was dependent on both P and K supply. Additionally, WUE was increased by e[CO2] and P and K supply. Both CO2 enrichment and K supply increased WUE by stimulating biomass accumulation and reducing water consumption. We concluded that the CO2-fertilization effect and WUE were dependent on both P and K supply. Less biomass accumulation in response to K supply in plants with P deficiency, indicate that a balanced nutrient status is crucial for crop production under e[CO2].