Cesium (Cs) in the environment is primarily absorbed by a potassium (K) transporter. OsHAK5 is a KT/HAK/KUP family K-transporter showing a high affinity for K. We created cultured rice cells whose OsHAK5 was knocked down by RNAi (named KD). In the medium containing 1.0 mM and less K, the growth of KD was significantly suppressed, suggesting that OsHAK5 greatly contributed to K absorption under limited K conditions. Although Cs suppressed the growth of KD and WT, stronger inhibition was observed on KD. Both KD and WT accumulated similar amounts of Cs when they were cultured in medium containing Cs, whereas lower amounts of K were detected in KD. These results suggest that OsHAK5 was less involved in the absorption of Cs, although it was essential to K absorption under limited K conditions. In contrast, this means that another transporter may contribute to cesium uptake in rice.