Geopolymers show high quasi-brittle behavior because of their ceramic-like characteristics. Recent findings have indicated that graphene can be used as an additive to improve the mechanical properties of composites. In this study, we report the effect of the addition of graphene nanoplatelets (GNPs) on the microstructure and mechanical properties of a fly ash based geopolymer. The GNPs are relatively homogeneously distributed in the matrix of all composites. However, overlapping and agglomerate formation of GNPs was detected by FESEM. The results showed that the compressive and flexural strength of the geopolymer improved by 1.44 and 2.16 times, respectively, when adding 1% GNPs. The introduction of a GNP filler, even at low filler weight fractions, increased the toughness, stress and strain at the first crack and rigidity. Moreover, the wettability decreased with an increase in GNP content.