The final thesis included the characterization of the welded joint of martensitic stainless steel X3CrNiMo13-4. The theoretical part of the thesis describes the basic types of stainless steels, with a particular focus on martensitic stainless steels, weldability, heat treatment, and microstructural changes in these steels. In the experimental part, the macrostructure and microstructure were analyzed, and various mechanical properties of the welded joint on a 50 mm thick plate delivered in a normalized state were examined. The structural analysis showed that the weld has a regular geometry and clearly defined passes. The base material has a fine-grained directional microstructure composed of ferrite and very fine carbides. In the heat-affected zone, the microstructure is martensitic, and the weld metal has a typical casting structure. Microstructural changes in the welded joint led to an increase in hardness compared to the base material, with the highest hardness values measured in the weld metal. In the heat-affected zone, higher toughness values were measured than in the weld metal. In the welded joint area, a higher tensile strength was measured compared to the guaranteed tensile strength value for the base material. No cracks were observed during bending tests on the critical surface of the weld face and root. The obtained results confirm the satisfactory quality of the welded joint.