Sodium uptake is a factor that determines potassium use efficiency in plants as sodium can partially replace potassium in plant cells. Rice (Oryza sativa) roots usually exclude sodium but actively take it up when the plant is deficient in potassium. In rice roots, a sodium transporter OsHKT2;1 mediates active sodium uptake. We previously revealed that variation in the expression of OsHKT2;1 underlies the variation in sodium accumulation between a low‐sodium‐accumulating indica cultivar, IR64, and a high‐sodium‐accumulating japonica cultivar, Koshihikari. In the present study, we evaluated IR64 and its near‐isogenic line IR64‐K carrying OsHKT2;1 and neighboring genes inherited from Koshihikari for grain yield. IR64‐K had a greater average grain yield and harvest index than IR64 in a pot culture experiment with three levels of potassium fertilizer. The differences were most significant under treatment without the potassium fertilizer. IR64‐K also showed a slightly higher grain yield than IR64 when grown in a paddy field without applying the potassium fertilizer. These results suggest that enhanced sodium uptake ability improves the grain yield of rice plants under low‐potassium‐input conditions.