Motivated by significant loss of mechanical properties during conventional fusion arc welding processes owing to under matching filler materials used and low efficiency associated with multipass welding, the viability of keyhole tungsten inert gas (K-TIG) welding for joining armour grade quenched and tempered (Q & T) steel was presented. Single pass full penetration was achieved on 9 mm thick plates at a speed of 28 cm/min −1 without using any filler materials and edge preparation. In-depth investigation into the weld was conducted by optical microscope, scanning electron microscope, electron back-scattered diffraction, microhardness and tensile test. The results show that the weld metal consists of dendritic structure and predominantly bainitic microstructure and is dominated by low angle grain boundaries. Hardness distribution across the weld is higher than current practice, which would lead to improved ballistic performance. Although the joint efficiency of the weld is 65% due to reduction in weld metal hardness, it is still much higher than that produced via conventional fusion welding, which is not surpassing 50%. It has been demonstrated that the K-TIG welding process offers a new way to weld medium thick armour grade Q & T steel with high efficiency and low cost, while maintaining the mechanical properties at a high level.