We have isolated and characterised AtCNGC10, one of the 20 members of the family of cyclic nucleotide (CN)-gated and calmodulin (CaM)-regulated channels (CNGCs) from Arabidopsis thaliana (L.) Heynh. AtCNGC10 bound CaM in a C-terminal subregion that contains a basic amphiphillic structure characteristic of CaM-binding proteins and that also overlaps with the predicted CN-binding domain. AtCNGC10 is insensitive to the broad-range K⁺ channel blocker, tetraethylammonium, and lacks a typical K⁺-signature motif. However, AtCNGC10 complemented K⁺ channel uptake mutants of Escherichia coli (LB650), yeast (Saccharomyces cerevisiae CY162) and Arabidopsis (akt1-1). Sense 35S-AtCNGC10 transformed into the Arabidopsis akt1-1 mutant, grew 1.7-fold better on K⁺-limited medium relative to the vector control. Coexpression of CaM and AtCNGC10 in E. coli showed that Ca²⁺ / CaM inhibited cell growth by 40%, while cGMP reversed the inhibition by Ca²⁺ / CaM, in a AtCNGC10-dependent manner. AtCNGC10 did not confer tolerance to Cs⁺ in E. coli, however, it confers tolerance to toxic levels of Na⁺ and Cs⁺ in the yeast K⁺ uptake mutant grown on low K⁺ medium. Antisense AtCNGC10 plants had 50% less potassium than wild type Columbia. Taken together, the studies from three evolutionarily diverse species demonstrated a role for the CaM-binding channel, AtCNGC10, in mediating the uptake of K⁺ in plants.