We report a novel approach to distributed optical fiber sensing that achieves strain-only measurement with no intrinsic temperature cross-sensitivity. This is accomplished by filling the microstructured cladding of a photonic crystal fiber with a simple gas-such as nitrogen-at constant pressure, passively compensating the thermo-optic effect of silica. The method was validated using a coherent Rayleigh-based φ-OTDR setup, demonstrating full strain sensitivity while effectively eliminating temperature-induced signal drift. The compensation mechanism is tunable and was achieved using a commercially available, non-specialized fiber under laboratory conditions. Limitations related to thermal range, stabilization time, and pressure control are discussed. Beyond strain sensing, this athermalization technique may significantly enhance the thermal stability of fiber interferometers and precision timing distribution over optical networks.