OSIRIS (Optical System for Imaging and low Resolution Integrated Spectroscopy) is the first light instrument of the Gran Telescopio Canarias (GTC). It provides a flexible and competitive tunable filter (TF). Since it is based on a Fabry-Perot interferometer working in a collimated beam, the TF transmission wavelength depends on the position of the target with respect to the optical axis. This effect is nonnegligible and must be accounted for in the data reduction. Our article establishes a wavelength calibration for OSIRIS TF with the accuracy required for spectrophotometric measurements using the full field of view (FOV) of the instrument. The variation of the transmission wavelength λ(R) across the FOV is well described by λ(R)=λ(0)/√{1 + R/(f2)}2, where λ(0) is the central wavelength, R represents the physical distance from the optical axis, and f2=185.70±0.17 mm is the effective focal length of the camera lens. This new empirical calibration yields an accuracy better than 1 Å across the entire OSIRIS FOV (˜8'×8'), provided that the position of the optical axis is known within 45 μm (≡1.5 binned pixels). We suggest a calibration protocol to grant such precision over long periods, upon realignment of OSIRIS optics, and in different wavelength ranges. This calibration differs from the calibration in the OSIRIS manual, which nonetheless provides an accuracy ≲1 Å for R≲2'.