Due to changes in the ecosystem and misuse of resources, salinity also increases. Approximately 20% of all irrigated land is affected by salinity and this will increase over time. Therefore, it is necessary to develop more environmentally friendly agricultural techniques but also to exploit potential crops with high nutritional value and tolerance to salinity like quinoa (Chenopodium quinoa Willd.). In this context, potassium is an essential macronutrient for plant growth and development. Furthermore, one of the strategies of some salt-tolerant plants is to increase the uptake of potassium under saline conditions such that the K+/Na+ ratio is maintained for a proper osmotic regulation in cells. Therefore, a study was conducted to investigate the effect of different concentrations of potassium (0.5, 2 and 6 mM K) on quinoa under different salinities (0, 100 and 200 mM NaCl). According to the results, an adequate supply of potassium under moderate salinity conditions benefited the plant growth, with a higher potassium uptake in the presence of salt. Under saline conditions, plant transpiration decreased significantly with a high correlation with stomatal density and a greater water use efficiency. Therefore, under saline conditions, adequate doses of potassium are highly recommended in quinoa cultivation. © 2021 The Authors. Journal of Agronomy and Crop Science published by Wiley-VCH GmbH.