Purpose <p>Soil K concentration greatly affects Cs uptake by plants, with high (low) levels of exchangeable K inhibiting (enhancing) Cs uptake. We investigated the effects of heterogeneous K concentrations in the rhizosphere on Cs uptake in rice.</p> Methods <p>Field experiments were conducted in Iwate, Japan (Experiment 1) and Fukushima, Japan (Experiment 2) to evaluate the effects of K fertilization on stable (¹³³Cs) and radioactive (¹³⁷Cs) Cs uptake, respectively. Three treatments were applied: KCl mixed with soil before transplantation (Mix_KCl), local application of KCl (L_KCl), and local application of slow-release K fertilizer (L_Kcoat).</p> Results <p>In Experiment 1, local K treatment resulted in heterogeneous K distribution; however, K uptake was comparable to that in Mix_KCl plots. ¹³³Cs uptake in L_Kcoat plots markedly increased compared with that in Mix_KCl plots, whereas that in L_KCl plots was higher, but not statistically different, compared with that in Mix_KCl plots. In Experiment 2, L_KCl and Mix_KCl treatments suppressed ¹³⁷Cs uptake with similar efficacy. The expression of most root K transport-related genes remained unaffected by local K application; however, some high-affinity K transport genes such as <i>OsHAK1</i> and <i>OsHKT2;1</i> were upregulated on the unfertilized side compared with those on the fertilized side under L_KCl.</p> Conclusions <p>Despite heterogeneous soil K concentrations, the effect on Cs uptake was small when rapid-releasing K was applied; the same amount of K applied using slow-release fertilizer accelerated Cs uptake. Therefore, the effect of local K application on Cs uptake by plants depends on the pattern and amount of K released during the rice-growing season.</p>