Potassium (K+) is an essential mineral nutrient for plant growth and development, with numerous membrane transporters and channels having been implicated in the maintenance and regulation of its homeostasis. The cation cesium (Cs+) is toxic for plants but shares similar chemical properties to the K+ ion and hence competes with its transport. Here, we report that K+ and Cs+ homeostasis in Arabidopsis thaliana also requires the action of ZIFL2 (Zinc-Induced Facilitator-Like 2), a member of the Major Facilitator Superfamily (MFS) of membrane transporters. We show that the Arabidopsis ZIFL2 is a functional transporter able to mediate K+ and Cs+ influx when heterologously expressed in yeast. Promoter–reporter, reverse transcription–PCR and fluorescent protein fusion experiments indicate that the predominant ZIFL2.1 isoform is targeted to the plasma membrane of endodermal and pericyle root cells. ZIFL2 loss of function and overexpression exacerbate and alleviate plant sensitivity, respectively, upon Cs+ and excess K+ supply, also influencing Cs+ whole-plant partitioning. We propose that the activity of this Arabidopsis MFS carrier promotes cellular K+ efflux in the root, thereby restricting Cs+/K+ xylem loading and subsequent root to shoot translocation under conditions of Cs+ or high K+ external supply.