A secondary battery is disclosed. In one aspect, the secondary battery includes an electrode assembly, a case accommodating the electrode assembly and a lead member extending from the electrode assembly to outside of the case. The secondary battery also includes an insulation member at least partially covering the lead member, the insulation member having an exterior portion that is located outside the case and an interior portion that is located inside the case, wherein the interior portion has a first thickness, and wherein at least part of the exterior portion has a second thickness less than the first thickness. The secondary battery may have a compact structure, and insulation characteristics of the secondary battery may be improved.