Thick dissimilar joints of steel and aluminum are still of high interest for shipbuilding in order to reduce the weight and the center of gravity of the ship. Thereby, a reduction of the CO2 emissions as a result of lower fuel consumption and a higher ship stability are achievable. The steel and aluminum ship parts are joined with the aid of an explosive-welded adapter, whose manufacturing is complex, time-consuming, and expensive. Furthermore, the adapter must be oversized to meet strength requirements. Therefore, the shipbuilding industry demands a better alternative. In this study, laser beam welding processes are developed for joining steel S355 J2 (t = 5 mm) with aluminum alloy AA6082-T651 (t = 10 mm) in a lap configuration using a laser beam source with a maximum output power of PL = 6 kW. Laser beam welding of this dissimilar material combination brings certain challenges, such as the formation of brittle microstructures in the weld metal depending on the aluminum content. To improve the microstructure and the associated mechanical properties of the weld seam, a filler material in the form of iron welding powder is used. The welding powder is provided in a groove in the aluminum bottom sheet. In this way, an iron-rich microstructure results, which leads to an increase in the weld seam quality, as shown in metallographic analysis and tensile tests. For example, the cross tension force can be increased by 100\%.