For joining of thick steel plates, commonly arc welding and partially laser-arc hybrid welding are used. Both techniques offer individual disadvantages besides their advantages. Arc welding processes are typically characterized by low welding speeds, high heat inputs, high distortions, and high filler material consumptions. Laser-arc hybrid welding processes are limited to weldable material thicknesses regarding weld imperfections as well as the ability to bridge gaps. Therefore, the investigations presented are about high-power diode laser beam welding of steel with plate thicknesses between 15 mm and 30 mm using output powers of up to 60 kW and welding speeds between 0.5 m/min and 1.0 m/min. The welding experiments contain butt welds by using weld backing materials. Among other things, influence of energy per unit length (laser beam power, welding speed), focal position, and plate thickness are analyzed for different reproducible processes. The evaluation of weld seams includes visual inspection, metallographic analyses regarding geometric characteristics, and weld imperfections as well as radiographic inspections. The investigations show achievable qualities and characteristics of the weld seams depending on the plate thickness by using high-power diode laser beam sources. Weld seams on plates with thicknesses of 15 mm and 22 mm were welded without cracks or other weld imperfections according to metallographic analyses and radiographic testing by using a laser beam power of 40 kW and the suitable process parameters developed. As a result, relatively high plate thicknesses can be welded in a single layer with high quality as well as comparatively high welding speed.