Provided are a cap assembly and a secondary battery including the same. The cap assembly includes: a cap plate; a vent plate located under the cap plate, including a vent that protrudes downward, and deformed when internal pressure of a secondary battery is increased; a sub-plate located under the vent plate, adhered to the vent, and constituting a current interrupt device (CID) along with the vent plate; a middle plate located between the vent plate and the sub-plate and electrically connected to the vent plate through the sub-plate; and an insulator located between the vent plate and the middle plate, wherein the middle plate includes a receiving groove in which the sub-plate is received. Accordingly, according to the inventive concept, a cap assembly having a novel structure that may reduce a resistance of a battery and increase output power may be provided. Also, according to the inventive concept, a multi-tab structure may be used without a risk of increasing a resistance of the battery. Also, according to the inventive concept, welding defects between the cap assembly and an electrode tab of an electrode assembly may be reduced.