The ability to securely disseminate data in a timely manner is critical to military missions within a hostile environment. Tactical wireless sensor networks WSN consist of power-constrained devices spread throughout a region-of-interest to provide data extraction in real time. In this thesis, we develop cyber security mechanisms to be implemented on a tactical WSN using the 6LoWPAN protocol for use by the United States Marine Corps USMC. Specifically, we develop an architectural framework for tactical WSNs by studying cyber security gaps and vulnerabilities within the 6LoWPAN security sublayer, which is based on the IEEE 802.15.4 standard. We develop a key management scheme and a centralized routing mechanism that is non-broadcast but feasible in an operational scenario. In addition, we modify the6LoWPAN enabled IEEE 802.15.4 frame structure to facilitate the newly developed keying and centralized routing mechanisms. Methods to aid in deployment planning are also discussed. The tactical WSN architecture was tested against a variety of well-known network attacks. The attacks simulated were spoofing, man-in-the-middle, and denial-of-service. Through MATLAB simulations, we showed the effectiveness and efficiency of the developed cyber security mechanisms to provide integrity and reliability to a deployed tactical WSN.