S960 and S700 are two new high strength steels recently developed by Tata Steel, UK. Due to the high power densities that can be achieved, small component distortion, and fast welding speeds, laser welding of thick section steels has been widely used in offshore construction and shipbuilding. However, the penetration depth is typically limited in single pass welding to 1–2 mm/kW laser power. Melt sagging is a typical defect for single pass autogenous laser welding of thick section materials. Multi-pass narrow gap laser welding techniques become more attractive because they can weld thicker sections of material with a moderate laser power and suppress the melt sagging problem. In addition, this approach requires less filler material, and the cumulative heat input to the material is reduced when compared with traditional arc welding techniques. However, there are many variables involved in this narrow gap laser welding technique, which makes this process more complicated than single pass autogenous laser welding. In this study, the effects of multi-pass ultra-narrow gap laser welding parameter interactions (i.e. laser power, welding speed and wire feed rate) on laser weld quality and the welding efficiency for S960 high strength steel plates were investigated. Moderate laser powers of 1 to 2 kW were used to weld S960 high strength steel plates with a very narrow parallel groove (1.2 mm). Statistical design of experiments was carried out to assess the process parameter interactions and to optimise the ultra-narrow gap laser welding parameters. Validation experiments indicate that the proposed models predict the responses adequately within the range of welding parameters that were used. Defect-free welds in 6 mm thick S960 steel were obtained with only two passes, using the optimised welding parameters, and these optimised parameters were successfully transferred to the welding of 8 mm thick S960 steel. In addition, they were also successfully transferred to the welding of 13 mm thick S700 steel with a small modification. The optimised narrow gap laser welded joints show almost the same tensile properties as the base material, with failures occurring in the base material away from the weld.