A secondary battery includes an electrode assembly formed by winding two different electrodes and a separator interposed therebetween, a can housing the electrode assembly, a cap assembly at a upper opening of the can; and a gasket interposed between the cap assembly and the can. The gasket has a relaxed condition prior to compression between the cap assembly and an upper portion of the can and a compressed condition after compression between the cap assembly and the upper portion of the can. The gasket includes a sidewall portion bent inwardly from outside of the can when the gasket is in the compressed condition, a bottom portion at a bottom of the sidewall portion and extending inwardly from a periphery of the can, a protruding portion on which the cap assembly is seated, and a concave portion formed between the protruding portion and an inner surface of the sidewall portion.