In this research work, the effect of seawater in the synthesis of fly ash based geopolymer was investigated. Fly ash as a binder was mixed with alkaline activator. The activator used was a mixture of NaOH solution and liquid sodium silicate. The NaOH solution was prepared by dissolving NaOH pellets in seawater to 10mol/L. In this study, compressive strength testing, X-ray diffraction, mercury intrusion porosimetry, scanning electron microscopy with energy dispersive spectroscopy, and analyses for acid- and water-soluble chloride contents were carried out on hardened geopolymer paste samples.