Provided is a separator having excellent safety and a lithium secondary battery including the same. The separator includes: a porous polymer substrate; and an organic/inorganic composite porous layer disposed on at least one surface of the porous polymer substrate and including inorganic particles and a binder polymer, wherein the inorganic particles have a BET specific surface area of 50-150 m2/g, the organic/inorganic composite porous layer has a thickness larger than the thickness of the porous polymer substrate, and the surface of the organic/inorganic composite porous layer has an arithmetic mean roughness of 100-500 nm.