Dissimilar lap joints of 1060 aluminum and T2 copper were obtained by friction stir spot welding (FSSW) at various rotational speeds. The measured process variables during welding revealed that an increasing rotational speed leads to a higher peak temperature but a lower torque and plunging force due to the greater softening degree of material. A continuous CuAl2-CuAl-Al4Cu9 laminated layer developed at the Al-Cu interface under the highest rotational speed, while the interface was characterized by the interruptedly distributed CuAl2 layer under lower rotational speeds. Intermetallic compound (IMC) formation sequence for CuAl2, CuAl and Al4Cu9 was predicted according to thermodynamic principles, which is consistent with the interfacial microstructure evolution identified by the transmission electron microscope (TEM). Besides the metallurgical bonding through the IMC layer at the Al-Cu interface, joints have a Cu hook extruded upward by the pin tool from the lower Cu sheet into the upper Al sheet, which provided additional mechanical interlocking between the sheets. Tensile properties of the joints were closely related to the thickness of the IMC layer and geometric features of the hook, higher tensile properties were found in joints owning a continuous interfacial IMC layer with a proper thickness as well as a high penetration depth of the hook into the upper Al sheet. •A higher rotational speed leads to a lower torque and plunging force.•CuAl2-CuAl-Al4Cu9 laminated layer forms at the hook interface at high heat input.•IMC formation sequence for CuAl2, CuAl and Al4Cu9 is predicted.•The joints are both metallurgical and mechanical bonded.•Hook morphology influences fracture path and shear strength.