Biological incorporation of cesium ions (Cs+) has recently attracted significant attention in terms of the possible applications for bioremediation of radiocesium and their significant roles in biogeochemical cycling. Although high concentrations of Cs+ exhibit cytotoxicity on microorganisms, there are a few reports on the promotive effects of Cs+ on microbial growth under K+-deficient conditions. However, whether this growth-promoting effect is a common phenomenon remains uncertain, and direct correlation between growth promotion and Cs+ uptake abilities has not been confirmed yet. Here, we validated the growth promotive effects of Cs+ uptake under K+-deficient conditions using an Escherichia coli strain with an inducible expression of the Kup K+ transporter that has nonspecific Cs+ transport activities (strain kup-IE). The strain kup-IE exhibited superior growth under the Cs+-supplemented and K+-deficient conditions compared to the wild type and the kup null strains. The intracellular Cs+ levels were significantly higher in strain kup-IE than in the other strains, and were well correlated with their growth yields. Furthermore, induction levels of the kup gene, intracellular Cs+ concentrations, and the growth stimulation by Cs+ also correlated positively. These results clearly demonstrated that Cs+ incorporation via Kup transporter restores growth defects of E. coli under K+-deficient conditions.