In mobile wireless sensor networks (MWSN) with a sparse node population, an end-to-end path may be broken frequently or may not exist at any time due to nodes' mobility, which makes it impossible to apply the existing shortest-path routing protocols based on static topology for MWSN. An RSSI-based opportunistic routing protocol for MWSN, named OR-RSSI, is proposed in this paper. In the proposed protocol, opportunistic probability (OP) based on the RSSI of the sink's beacon packets and mobility vector (MV) is established and the best node with the highest OP available at that instant is used to store and relay packets at each hop after packets are broadcasted. Simulation results show that OR-RSSI is more feasible for sparse MWSN and performs better on successful delivery ratio, good put and energy overhead as compared to traditional TinyAODV.