ISTTOK is equipped with two probe systems that allow the simultaneous measurement of the three-dimensional characteristics of the edge fluctuations with high temporal resolution. Electrostatic fluctuations consistent with the geodesic acoustic mode (GAM) are observed in the edge plasma. The radial, poloidal and toroidal structure of the fluctuations are investigated and good agreement with the GAM theoretical predictions is found. Furthermore, experimental evidence is presented suggesting that the GAM is modulating long-range correlations and the ambient turbulent fluctuations.