This work evaluates the resistance of alkali-activated slag (AAS) mortar against sodium sulfate attack. The effects of immersion in 5% sodium sulfate solution under room temperature and wetting-drying cycles on the compressive strength of mortar specimens were considered for evaluating the extent of degradation. Mortar specimens prepared from type II and V Portland cements (PC2 and PC5) in accordance with ASTM standard were also used as reference. To characterize the chemical products of the degradation process due to sodium sulfate attack, the specimens were also studied by X-ray diffractometry, scanning electron microscopy, and the elemental analysis by energy dispersive X-ray spectroscopy. After 360 days of exposure to the sodium sulfate solution, PC2, PC5 and AAS cements showed 71, 52 and 45% reduction in compressive strength, respectively. According to the obtained results, AAS cement exhibits a higher sulfate resistance compared to PC2 and PC5.