As one of the most well-established biocompatible transition metal nitrides, titanium nitride (TiN) has been widely applied for fiber waveguide coupling device applications. This study proposes a TiN-modified fiber optic interferometer. Benefiting from the unique properties of TiN, including ultrathin nanolayer, high refractive index, and broad-spectrum optical absorption, the refractive index (RI) response of the interferometer is greatly enhanced, which is desired all the time in the field of biosensing. The experimental results show that the deposited TiN nanoparticles (NPs) can enhance the evanescent field excitation and modulate the effective RI difference of the interferometer, which eventually results in the RI response enhancement. Besides, after incorporating the TiN with different concentrations, the resonant wavelength and the RI responses of the interferometer are enhanced to varying degrees. Benefitting from this advantage, the sensing performances, including sensitivity and measurement range, can be flexibly adapted based on different detection requirements. Since RI response can effectively reflect the detection ability of biosensors, the proposed TiN-sensitized fiber optic interferometer can be potentially applied for high-sensitive biosensing applications.