A method and an apparatus for allocating a flexible transmission slot in a wireless local area network (LAN) system are disclosed. A flexible transmission slot allocation method of an access point (AP) in a wireless local area network (WLAN) system according to an exemplary embodiment includes transmitting a beacon including a traffic indication map (TIM) bit to a station, receiving a power save poll (PS-Poll) from the station in a slot implicitly allocated by the TIM bit, and transmitting an acknowledgement (ACK) including transmission slot allocation information on downlink data to the station.