The phenomenon of liquid metal embrittlement (LME) poses safety concerns for welded joints in the manufacturing field. In present study, LME was observed in refill friction stir spot welding (refill FSSW) of dissimilar magnesium (Mg) to galvanized steel. This marks the first reported proof of evidence of LME in the field of solid-state welding. Microstructural characterization of cracks formed during the welding process revealed typical characteristics of LME, specifically the penetration and enrichment of Zn at the Mg alloy grain boundaries and the formation of a liquefied phase. Tensile tests of Zn-coated Mg alloy were conducted at elevated temperatures to validate the LME phenomenon in refill FSSW and to identify the temperature range in which LME occurs. Based on these observations, a mechanism of LME formation for the Mg-Zn system in refill FSSW is proposed. Additionally, strategies to prevent LME are suggested and experimentally validated.