The article presents research on the laser beam welding of 10 mm thick T-joints made of thermomechanically worked high-strength steel S700MC without using a filler metal. The research-related tests involved making single-sided and double-sided welded joints as well as performing non-destructive tests. The quality of joints satisfied the requirements of quality level B according to the PN-EN ISO 13919-1 standard. The single-sided welding performed using a beam power of 11 kW enabled the obtainment of 8 mm deep penetration without noticeable displacements in the web. The double-sided welded joints were characterized by correct geometry; the dimensions of pores present in the weld metal satisfied the maximum pore size criterion specified for quality level B. The weld microstructure was bainitic-ferritic; the hardness of the weld was by about 60 HV1 higher than that of the base material (280 HV1). The HAZ revealed a small decrease in hardness in comparison with that of the base material.