Potassium (K) and magnesium (Mg) are essential macronutrients for plants; they play crucial roles for photoassimilate production and transport. The knowledge on both individual and interactive effects of K and Mg nutrition in potato (Solanum tuberosum L.) is limited. We aimed to determine whether K or Mg deficiencies impair photoassimilate production and transport, and consequently the development of tubers which are strong sink organs for photoassimilates. Potato plants were grown in pots using sand culture under various K and Mg supplies. Biomass production, CO2 net assimilation, leaf sugar concentrations and transcript levels of H+/sucrose symporters in leaves were measured. Both K and Mg deficiencies reduced CO2 net assimilation and biomass production, with stronger reductions during K deficiency. Sugars accumulated in K‐ and, more importantly, in Mg‐deficient leaves. Low K or Mg supplies resulted in increased transcript levels of H+/sucrose symporters, but the increase was less pronounced during Mg deficiency. The lower increase of transcript levels of H+/sucrose symporters under Mg deficiency was probably caused by an impaired sucrose transport already at an earlier step, namely the efflux of sucrose from mesophyll cells into the apoplast. Thus, we assume that K and Mg deficiencies caused sugar accumulation in separated cell compartments of source leaves leading to a different impact on the gene expression of sucrose transport systems. Tuber sugar and starch concentrations, however, remained unaffected under the various treatments. Nevertheless, the total amount of tuber sugar and starch per plant decreased significantly during K and Mg deficiencies.