Routing Protocol for low power and Lossy networks (RPL) is a standardized routing protocol for low power and lossy networks (LLNs) such as Internet of Things (IoT). RPL is designed to be a simple (but efficient) and practical networking protocol to perform routing in scalable IoT networks that consists of thousands of resource constrained devices. These tiny intercommunicating devices are currently in use in a large array of IoT application services (e.g., eHealth, smart agriculture, smart grids, and home automation). However, the lack of scalability, the low communication reliability, and the vulnerability towards various security threats still remains significant challenges in the broader adoption of RPL in LLNs.In this paper, we propose RECOUP, a reliable group communication routing protocol for IoT networks. RECOUP efficiently uses a low-overhead cluster-based multicast routing technique on top of the RPL protocol. RECOUP increases the probability of message delivery to the intended destination(s), irrespective of the network size and faults (such as broken links or nonresponsive nodes), and in the presence of misbehaving nodes. We show that the cluster-based routing mechanism of RECOUP remains robust in presence of various topology (i.e., rank and sybil) and data communication (i.e., blackhole, wormhole, and jamming) attacks targeting the IoT networking infrastructure. An implementation of RECOUP is realized in Contiki. Our results show the effectiveness of RECOUP over state-of-art protocols concerning packet delivery ratio to 25%, end-to-end delay down to 100 ms, low radio transmissions required for per packet delivery to 6 mJ, and most importantly, it improves the robustness and scalability of data communication process in the whole network.