The main objective of this research work was to understand the different challenges related to hybrid laser-arc welding (HLAW) of thick gauge section assemblies of low carbon 13%Cr-4%Ni martensitic stainless steel and develop a practical solution by adapting and optimizing this relatively new welding process in order to attain higher processing efficiency through a reduction in the number of welding passes necessary to fill the groove gap. Also a special focus was given to the development of the hybrid and tandem laser-arc welding techniques for the root pass. In this study, the processing methodology using the hybrid/tandem laser-arc welding technology was also adapted based on the thickness of the low carbon martensitic stainless steel plates, namely a single pass HLAW process for a 10-mm thick section and a multi-pass hybrid/tandem laser-arc welding process for a 25-mm thick section. After welding, the joint integrity was evaluated in terms of microstructure, defects and mechanical properties in both the as-welded and post-weld tempered conditions. The effect of different welding speeds on the as-welded joint integrity of the 10-mm thick and 25-mm thick assemblies was characterized in terms of the weld bead geometry, defects, microstructure, hardness, ultimate tensile strength and impact energy. Significant defects such as porosity, root humping, underfill and excessive penetration were observed at a low welding speed of 0.5 m/min. However the welds met the specifications of ISO 12932 at a speed higher than 0.75 m/min. The ultimate tensile strength and Charpy impact energy values of the fully penetrated welds in the tempered condition were acceptable according to ASTM, ASME and industrial specifications, which show good potential for introducing hybrid/tandem laser-arc welding technology for the manufacturing of next generation hydroelectric turbine components.