<p>Arbuscular mycorrhizal (AM) fungi improve water and nutrient acquisition of most land plants. Additionally, they can help plants to alleviate abiotic stresses, such as salinity which causes a major threat for many crop species. Potassium (K⁺) plays a major role in plant tolerance to salinity, and we recently demonstrated, by using rubidium (Rb⁺) as a proxy, that AM fungi can directly transfer K⁺ to their host plant. Here, we first investigated the impact of K⁺ availability on soybean root development upon salinity. Then, using two-compartment systems, we also inoculated soybean plants with the AM fungus <i>Rhizophagus irregularis</i>, grew them in various K⁺ and sodium (Na⁺) regimes, and used Rb⁺ to track K⁺ movements. Root development parameters, biomass, colonization rate, and nutrient concentrations were assessed in AM and non-mycorrhizal plants. Our results show that soybean root development was significantly affected by NaCl treatments, rather than K⁺ availability. Additionally, although the AM symbiosis was drastically reduced by high salinity, it improved K⁺ concentrations and prevented Na⁺ accumulation in inoculated plants, mainly under limiting K⁺ conditions. Rb⁺ transport was observed only when the plants were in demand for K⁺, but was inhibited by high salinity. Finally, we also show that the addition of NaCl slightly influences the availability of K⁺ and Rb⁺. This report shows the combined impact of K⁺ availability and AM symbiosis on soybean tolerance to salinity and discusses the limitations of using Rb⁺ as a proxy for K⁺ upon increasing salinity conditions.</p>