Disclosed is a secondary battery. According to the present invention, a charging/discharging pin is prevented from being separated from a top cap while the secondary is charged and discharged to improve safety during the charging/discharging of the secondary battery. According to an aspect of the present invention, the secondary battery includes a can accommodating an electrode assembly, a top cap provided on an upper portion of the can, and a groove defined in a surface of the top cap and having a shape that is recessed downward. When the top cap is viewed from an upper side, the groove has a closed curve shape.