A simple aqueous‐phase synthesis of PbSe nanocubes with tunable sizes has been developed by first preparing a Na2SeSO3 stock solution through dissolution of selenium powder in a solution of Na2SO3 at 90–100 °C for 30 min, and adding part of this solution to a mixture of lead acetate and acetic acid at room temperature with stirring for only 5–8 min to complete the nanocrystal growth. Adjusting the volume of acetic acid and Na2SeSO3 solution added enabled the size of the nanocrystals to be tuned, with average edge lengths of 13 to 121 nm attained. Changes in solution color revealed very different crystal growth rates for the 13 and 121 nm nanocubes. The PbSe cubes exhibit size‐dependent absorption bands in the ultraviolet and visible‐light region; the band positions show progressive redshifts with increasing particle size. Slight photocatalytic activity upon 532 nm laser irradiation of the nanocubes suggests the presence of higher energy levels in the band structure of PbSe. The synthetic conditions can be easily scaled up to obtain a large quantity of PbSe nanocubes for applications.