We present a topology discovery service for software-defined wireless multi-hop networks, which is capable of capturing rich information about the network topology, such as link quality, interference and node characteristics etc., to effectively support the requirements of higher-layer SDN applications such as routing, channel allocation and transmission power control with enhanced flexibility. We analyse the general topology representation required by such SDN applications and specify the procedure and mechanism required at the controller and nodes to maintain this representation. A proof-of-concept implementation of our service on an SDN enabled multi-channel multi-interface wireless multi-hop network testbed shows the feasibility of our proposal.