Regulation of root transport systems is essential under fluctuating nutrient supply. In the case of potassium (K+), HAK/KUP/KT K+ transporters and voltage‐gated K+ channels ensure root K+ uptake in a wide range of K+ concentrations. In Arabidopsis, the CIPK23/CBL1‐9 complex regulates both transporter‐ and channel‐mediated root K+ uptake. However, research about K+ homeostasis in crops is in demand due to species‐specific mechanisms. In the present manuscript, we studied the contribution of the voltage‐gated K+ channel LKT1 and the protein kinase SlCIPK23 to K+ uptake in tomato plants by analyzing gene‐edited knock‐out tomato mutant lines, together with two‐electrode voltage‐clamp experiments in Xenopus oocytes and protein‐protein interaction analyses. It is shown that LKT1 is a crucial player in tomato K+ nutrition by contributing ~50% to root K+ uptake under K+‐sufficient conditions. Moreover, SlCIPK23 was responsible for ~100% of LKT1 and ~40% of the SlHAK5 K+ transporter activity in planta. Mg+2 and Na+ compensated for K+ deficit in tomato roots to a large extent and the accumulation of Na+ was strongly dependent on SlCIPK23 function. The role of CIPK23 in Na+ accumulation in tomato roots was not conserved in Arabidopsis, which expands the current set of CIPK23‐like protein functions in plants. This article is protected by copyright. All rights reserved.