Abstract: For the friction stir spot welding (FSSW) of Mg-Al dissimilar metals, the joint strength is deteriorated due to the formation of intermetallic compound (IMC) layer at the interface. In the present study, AZ31 Mg and 2024 Al sheets are friction stir spot welded with and without the addition of a Zn interlayer 0.1 mm in thickness. The results show that the load of the Mg-Al FSSW joint without the Zn interlayer is only 0.8 kN due to the mechanical bonding between Mg and Al substrates as well as the formation of 5 μm thick IMC layer at the interface. For the joints with Zn interlayer, for the region away from the keyhole in the hook region, the Zn interlayer reacts with the substrates, forming the Al-Zn zone and Zn-Mg zone, thereby eliminating the formation of Mg-Al IMCs. In addition, the Zn interlayer reacts with the substrates, reducing the IMC layer thickness to about 2 μm next to the keyhole in the hook region. As a result, the joint load increases from 0.8 kN to 1.4 kN with a increase of 75%.