This paper aims to investigate the behavior of fly ash/slag geopolymer mortar (FA/S-GM) exposed to sulfate attack. Four FA/S-GM mixtures with different slag replacement levels (0 - 20% by mass of FA) were prepared. Four more ordinary Portland cement/slag mortar (OPC/S-M) mixtures with the same slag replacement levels were prepared as control specimens. All mortar specimens were immersed in 5% sodium sulfate solution for 52 weeks. Compressive strength, Expansion, weight gain, x-ray diffraction (XRD), and scan electron microscopy (SEM) approaches were conducted throughout this research. The mechanical and physical properties of FA/S-GM were superior to those of OPC/S-M, revealing excellent sulfate resistance. After 52 weeks of sulfate exposure, the compressive strength of FA/S-GM increased while the compressive strength of OPC/S-M decreased when compared to their pre-sulfate strength. FA/S-GM and OPC/S-M had optimal slag contents of 20% and 10%, respectively. XRD and SEM analyses revealed no ettringite or gypsum in FA/S-GM.