Novel rhombic dodecahedral SnS nanocrystals were prepared via a facile one-pot hydrothermal method for the first time, which exhibit a large extinction coefficient of 36.8 L g-1 cm-1 and a high photothermal conversion efficiency of 39.4% under irradiation with a 785 nm laser. Moreover, they show good performance for photothermal therapy of HeLa tumors.