Abstract The dynamic slip behavior of faults is strongly influenced by the thermal and hydraulic properties of pore fluids, which control thermal pressurization (TP) during seismic slip. While previous experiments showed significant TP‐induced weakening in DI‐water‐saturated Groningen sandstone gouges, in situ fluids are brine or brine–gas mixtures with different properties than DI water. We performed intermediate‐velocity (0.05 m/s) rotary‐shear experiments on simulated gouges with four pore fluids (water, brine, 1, and 5 cSt silicone oils) under undrained conditions. TP dominated weakening in all cases, but fluids with higher thermal expansivity (brine) and lower hydraulic diffusivity (5 cSt oil) produced less weakening than water and 1 cSt oil. Model comparisons showed improved agreement when considering fluid‐dependent permeability changes. These findings underscore the role of fluid properties in TP efficiency and the importance of evolving gouge permeability. This work refines TP understanding and provides experimental constraints on TP‐induced weakening in fluid‐saturated gouge‐filled faults.