<p>High-strength steel is an important lightweight material for the aerospace and offshore structures industry. Hybrid laser-arc welding has been a potential alternative manufacture technique for welding thick-section structural components. Thus, in this work, we assess the microstructure and mechanical properties of a 12-mm thick-section high-strength steel welded joint obtained by multi-pass hybrid fiber laser-arc welding. The results showed that welded joints without cracks and lack-of-fusion defects were obtained by multi-pass hybrid fiber laser-arc welding with backing layer, filling layer and covering layer. The base metal was composed by massive polygonal ferrite and small granular carbides, while the weld metal consisted of massive columnar grains of martensite. The results also showed that the dislocation density was the main reason for the difference of the microhardness of the weld metal of the backing layer, filling layer and covering layer. Meanwhile, the average yield strength and the average ultimate tensile strength of the welded joints were 1010 and 1043&#xa0;MPa, respectively. The average elongation of the welded joint was 3.4%. Fracture occurred in the base metal instead of the weld metal. High tensile strength martensite and high density dislocation were the two key factors for the high strength of the weld metal. Additionally, the impact toughness of the weld metal was the lower than that of the heat-affected zone due to the high impact absorbing energy of the ferrite that existed in the heat-affected zone.</p> Graphical Abstract <p></p>