Friction Stir Welding (FSW) is a solid-state process of welding based on deformation and atomic diffusion. The joint relies on the application of stresses above the yield strength of the material at a high temperature below the melting point of the workpiece. This produces the deformation and subsequently joining of the metal, or metals in contact. It can be used over a wide array of material systems if performed correctly due to its advantages over the other joining processes. Some advantages of the process are the elimination of hydrogen induce cracking and weld solidification cracking, and the outstanding mechanical and metallurgical properties that can be obtained. Also, FSW does not require filler metal, so lightweight joints can be achieved. More importantly, it is possible to reduce the softening during welding in the armor material due to the extension of the head affected zone (HAZ) and high energy input, especially in arc-based welding processes. For all this, FSW is becoming an important process in the armor industry. In this work, Rolled Homogenous Armor (RHA) and High Hardness Armor (HHA) Steels were individually joined using an RM15 Bond Technologies FSW machine. Process development in plates of ╝ in thickness was done using position and force control options to find a free defect window. Some experiments were also conducted using thermal management during welding by implementing a coil and liquid nitrogen. Then, some of the plates were metallographically characterized using optical microscopy and scanning electron microscopy. Also, microhardness map testing was performed and related to the weld nugget. Finally, RHA was submitted to ballistic testing. It was possible to find a free defect window for both RHA and HHA steels. It was also found that thermal management helps to reduce the over-tempering of the material during the welding, however, more experiments should be run. On the other hand, hardness mapping showed a narrow HAZ with smaller results of hardness than in the nugget and base material. Finally, a V50 of 455 m/s was obtained in the ballistic testing of RHA. This work was able to find suit process parameters for both RHA and HHA steel by FSW. According to the metallographic and hardness results, softening is still present in the HAZ. However, the ballistic performance found for the RHA is better than that of arc-based welding processes.