Exemplary embodiments of the present disclosure provide a separator for a secondary battery, capable of preventing heat from transferring to an adjacent region in an electrode even when non-ideal heat is generated by an electrode active material, a trigger point of thermal runaway. A separator for a secondary battery according to the present disclosure includes a porous polymer substrate and a ceramic layer provided on at least one surface of the porous polymer substrate. The ceramic layer includes a ceramic composite. The ceramic composite includes a hollow portion, a ceramic shell surrounding the hollow portion, and a doping material dispersed in the ceramic shell. The hollow portion is in a vacuum state.