We conducted experiments to investigate the influence of pore fluid pressure on the frictional strength and slip behavior of gouge bearing faults. Saw‐cut porous sandstone samples with a layer of gouge powders placed between the pre‐cut surfaces were deformed in the conventional tri‐axial loading configuration. A series of velocity‐step tests were performed to measure the response of the friction coefficient to variations in sliding velocity. Pore volume changes were monitored during shearing of the gouge. Our results demonstrate that, under constant effective pressure, increasing pore pressure stabilizes the frictional slip of faults with all four gouge materials including antigorite, olivine, quartz and chrysotile. The stabilizing effect is the strongest in antigorite gouge, which shows an evolution of friction parameters from velocity‐weakening towards velocity‐strengthening behavior with increasing pore pressure. Experiments with controlled pore volume show that that the pore volume reduction diminishes under high pore fluid pressures, implying an increasing dilation component at these conditions. The dilatant hardening mechanism can explain the observed strengthening. These results provide a possible explanation to the observed spatial correlation between slow slip events and high pore pressure in many subduction zones.