We investigate a thermally tunable integrated narrow-bandwidth interference filter for light in the infrared wavelength region around γ = 1550 nm fabricated in a channel waveguide in lithium niobate. The filter is based on a holographically recorded and thermally fixed refractive-index Bragg grating. The device is connected to optical single-mode fibers and works polarization-independent. We measure a peak reflectivity of 95% and a linewidth (FWHM) of 0.09 nm. By temperature-controlling the peak wavelength of the filter is tuned. For the linear thermal expansion coefficient of lithium niobate along the c axis the value α33 = (4.5 ± 0.5) × 10-6 K-1 is obtained. Permanent illumination of the filter with incoherent light allows us to adjust the peak reflectivity.