A secondary battery including an electrode assembly including a positive electrode including a positive electrode active material layer, a negative electrode including a negative electrode active material layer, a separator separating the positive and negative electrodes from each other, and an electrolyte. The separator includes a porous layer comprising a ceramic material and a binder, and a polyolefin-based resin layer. The porous layer has a centerline average roughness (Ra) of 0.3 μm to 1.5 μm, the polyolefin-based resin layer has a porosity of 30% to 60%, and the polyolefin-based resin layer has a compressibility of 4% to 10%.