Wireless sensor-actuator networks have a big usage potential in numerous industrial use cases. They allow easy and flexible deployment of nodes for monitoring and controlling various industrial applications as for example the supervision of critical infrastructures or monitoring and control in factory and process automation. However, missing or weak security of wireless communications would restrain the acceptance of wireless sensor-actuator networks in industrial settings. Security measures are crucial to ensure a reliable operation that is robust to accidental and targeted attacks. This paper describes the security architecture specially crafted for industrial usage environments. It provides mandatory hop-by-hop frame protection as well as authentication, access control, and protection of end-to-end communication. A secure wake-up scheme prevents a certain class of Denial-of-Service attacks, and a secure cooperative MIMO scheme improves communication reliability.