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 the active sodium uptake. We previously revealed that variation in the expression of OsHKT2;1 underlie the variation in sodium uptake between a low-sodium-uptake indica cultivar, IR64, and a high-sodium-uptake japonica cultivar, Koshihikari. In the present study, we evaluated IR64 and its near-isogenic line IR64-K that carrying OsHKT2;1 and neighboring genes inherited from Koshihikari for grain yields. 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 potassium fertilizer. IR64-K also showed a slightly higher grain yield than IR64 when grown in a paddy field without potassium fertilizer application. These results suggest that the enhanced ability of sodium uptake improves grain yields of rice plants under low-potassium-input conditions.