According to the present invention, a method by which a terminal transmits a buffer status report (BSR) in a wireless communication system comprises the steps of: receiving, from a base station, a message including information for a BSR request; and triggering the BSR on the basis of the information; and transmitting the BSR to the base station. The triggered BSR is a periodic BSR, and the triggering step causes a timer for the periodic BSR to expire. The information for a BSR request is indicated by a polling field included in a medium access control (MAC) sub-head.