The non-essential cation caesium (Cs(+)) is assimilated by all organisms. Thus, anthropogenically released radiocaesium is of concern to agriculture. Cs(+) accumulates owing to its chemical similarity to the potassium ion (K(+)). The apparent lack of a Cs(+)-specific uptake mechanism has obstructed attempts to manipulate Cs(+) accumulation without causing pleiotropic effects. Here we show that the SNARE protein Sec22p/SEC22 specifically impacts Cs(+) accumulation in yeast and in plants. Loss of Saccharomyces cerevisiae Sec22p does not affect K(+) homeostasis, yet halves Cs(+) concentration compared with the wild type. Mathematical modelling of the uptake time course predicts a compromised vacuolar Cs(+) deposition in sec22Δ. Biochemical fractionation confirms this and indicates a new feature of Sec22p in enhancing non-selective cation deposition. A developmentally controlled loss-of-function mutant of the orthologous Arabidopsis thaliana SEC22 phenocopies the reduced Cs(+) uptake without affecting plant growth. This finding provides a new strategy to reduce radiocaesium entry into the food chain.