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High‐viscosity fluids are often used during hydraulic fracking operations in geo‐reservoirs. Here, we performed dedicated experiments to study the influence of fluid viscosity on fault reactivation and associated induced earthquakes. Experiments were conducted in the rotary‐shear machine SHIVA on experimental fault of Westerly Granite saturated by fluids with increasing viscosity (at room temperature) from 0.1 mPa.s (water) to 1.2 Pa.s (99% glycerol). Fault reactivation was triggered at constant effective normal‐stress by increasing the shear stress acting on the fault. Our results showed that independent of the viscosity, fault reactivation followed a Coulomb‐failure criterion. Instead, fluid viscosity affected the fault weakening mechanism: flash‐heating was the dominant weakening mechanism in room‐humidity and water‐saturated conditions, whereas the presence of more viscous fluids favored the activation of elasto‐hydrodynamic lubrication. Independent of the weakening mechanism, the breakdown work Wb dissipated during seismic faulting increased with slip U following a power law (Wb ∝ U 1.25) in agreement with seismological estimates of natural and induced earthquakes.