Ocean warming has facilitated the extension of Heliocidaris crassispina to Oga Peninsula, Japan, where the native species Mesocentrotus nudus has disappeared. To verify the temperature impacts on the physiology and behaviour of the two species, we reared small sea urchins at the increasing–decreasing temperature rate of 2.5 °C·week −1 . The righting response, lantern reflex, gonad and gut carbon (C) and nitrogen (N) contents, and feeding rate were investigated. The high and low temperature limits of H. crassispina were 33.3 and 3.9 °C, respectively, which were higher than those of M. nudus. The optimal temperature ranges for behaviour and feeding in H. crassispina were 10.3–31.0 and 10.3–33.4 °C, respectively, which were higher than those in M. nudus. Feeding rates decreased significantly in both species when the temperature approached the high or low temperature limit, but the gut C and N contents of were not greatly affected. At 26–31 °C, the feeding rate significantly decreased in M. nudus but not in H. crassispina, which may explain the replacement of M. nudus by H. crassispina in the Oga Peninsula.