An in situ study of the spatial distribution of the piezoelectric properties of an LiNb(1−x)TaxO3 crystal (⟨x⟩ = 0.088) in an external DC electric field was performed by the method of X-ray diffraction of synchrotron radiation at angles close to 180°. The variation of the d22 piezoelectric coefficient was determined with high accuracy (up to 2%) in local areas by the angular shift of the diffraction maximum under the action of the electric field (E = 1582 V mm−1) applied on the crystal along the Y axis, during scanning along the X and Z directions. A correlation between the piezoelectric strain amplitude and the concentration of tantalum obtained by X-ray fluorescence mapping has been established.