The effect of tool rotational speed and double-sided welding in friction stir welding of aluminum alloy AA5083 and GL A36 steel was investigated. Defect-free joints were obtained in single- and double-sided welding using a rotational speed of 300 rpm. The increase in rotational speed or the application of double-sided welding increased the amount of dispersed steel particles in the aluminum stir zone, which stimulated void formation in the joint. In spite of the grain refinement in the stir zone, hardness is similar for all weld zones of the aluminum alloy, increasing significantly just in the steel near the weld interface due to severe work hardening. Besides this, the studied rotational speeds and double-sided welding did not exert any significant influence on the tensile strength of the joints, despite that a symmetric joint configuration with the welding through the full material thickness is guaranteed in the double-sided welding. The formation of intermetallic compound (IMC) layers is observed due to interdiffusion in the aluminum/steel interface. Investigations via scanning electron microscopy and energy-dispersive X-ray spectroscopy reveal that the IMC layer with 300–400 nm thickness is composed of Fe₂Al₅ or FeAl₃ throughout the entire weld interface.