Internet of Things environments are comprised of heterogeneous devices that are continuously exchanging information and being accessed ubiquitously through lossy networks. This drives the need of a flexible, lightweight and adaptive access control mechanism to cope with the pervasive nature of such global ecosystem, ensuring, at the same time, reliable communications between trusted devices. To fill this gap, this paper proposes a flexible trust-aware access control system for IoT (TACIoT), which provides an end-to-end and reliable security mechanism for IoT devices, based on a lightweight authorization mechanism and a novel trust modelthat has been specially devised for IoT environments. TACIoT extends traditional access control systems by taking into account trust values which are based on reputation, quality of service, security considerations and devices’ social relationships. TACIoT has been implemented and evaluated successfully in a real testbed for constrained and non-constrained IoT devices.