Wireless sensor networks are collections of large number of sensor nodes. The sensor nodes are featured with limited energy, computation and transmission power. Each node in the network coordinates with every other node in forwarding their packets to reach the destination. Since these nodes operate in a physically insecure environment; they are vulnerable to different types of attacks such as selective forwarding and sinkhole. These attacks can inject malicious packets by compromising the node. Geographical routing protocols of wireless sensor networks have been developed without considering the security aspects against these attacks. In this paper, a secure routing protocol named secured greedy perimeter stateless routing protocol (S-GPSR) is proposed for mobile sensor networks by incorporating trust based mechanism in the existing greedy perimeter stateless routing protocol (GPSR). Simulation results prove that S-GPSR outperforms the GPSR by reducing the overhead and improving the delivery ratio of the networks.