Laser spot welding was used for the dissimilar joining of aluminum alloy to steel in the keyhole mode. The results showed that the defocusing amount and laser power had significant influence on the weld formation. The aluminum alloy was prone to the formation of welding defects, such as porosity and cracks in the fusion zone, which resulted in an extremely instable welding process and poor joint strength. In order to improve the weld quality, a copper heat sink was placed under the aluminum alloy, which effectively absorbed the heat of the aluminum alloy and reduced the previous reported welding defects. The influence of laser power, defocusing amount, welding time, and shielding gas on the joint characteristics was also investigated. The microstructure analysis showed that a conelike fusion zone and sparsely distributed Fe-Al intermetallic compounds were formed. Hence, a combination of metallurgical bonding and mechanical interlocking was achieved in the laser keyhole spot welds, enhancing the joint mechanical properties. The joint strength of the laser keyhole spot welds reached 620 N/mm, which was comparable to that obtained in resistance spot welds.