The next generation of electric grid has a scalable and reliable bi-directional communication infrastructure known as Advanced Metering Infrastructure (AMI) to allow monitoring and controlling of grid resources. In this kind of communication system, comprising thousands of resource-constrained nodes such as smart meters, the routing protocol plays a critical role to guarantee reliability and low latency for data delivery. The Routing Protocol for Low Power and Lossy Networks (RPL) currently is standardized by IETF which is expected to meet the requirements of AMI networks as the standard routing protocol. On the other hand, cooperative routing has gained a lot of attention recently. Cooperative routing improves performance over traditional routing by exploiting the broadcasting nature of wireless channels. Our objective in this paper is to propose a cooperative approach to RPL for application in AMI networks. Our proposed protocol, termed as Coop-RPL, is specially designed for the challenges of AMI networks. Coop-RPL ensures reliable packet transmission by selecting an optimal relay node. Performance evaluation demonstrates the efficiency of Coop-RPL for AMI networks with the aim of achieving increased PDR, and reduced number of retransmissions and end-to-end delay.