Provided are a method and device for processing an uplink (UL) signal in a WLAN system. An access point (AP) receives a request to send (RTS) frame from one station (STA) among a plurality of STAs, transmits a group identifier (ID) of a group to which STAs for performing UL multi user multiple-input multiple-output (MU-MIMO) transmission together with the one STA, among group IDs of a group to which the one STA belongs, and receives UL signals from the STAs which perform the UL MU-MIMO transmission such that the received UL signals are processed.