The incommensurately modulated structure of Sr(2)Nb(2)O(7) at room temperature is reported, as determined by single-crystal X-ray diffraction. The crystal structure of Sr(2)Nb(2)O(7) comprises slabs with a perovskite-type structure that are separated by planes of additional O atoms. The driving force for the modulation is shown to be internal strain around the Sr atoms that lie at the interface between the slabs. At room temperature, Sr(2)Nb(2)O(7) crystallizes in the superspace group Cmc2(1)(alpha00)0s0, with lattice parameters a = 3.9544 (7), b = 26.767 (6) and c = 5.6961 (8) A. The modulation wave vector is determined as q = 0.488 (3) a(*). X-ray diffraction data were collected at a synchrotron using a CCD area detector. A total of 3626 unique main reflections and 1262 unique first-order satellites with I > 3sigma(I) were obtained. Refinements using a single harmonic modulation wave converged at R = 0.057 (R = 0.051 for the main reflections and R = 0.121 for the satellite reflections). The modulated structure is interpreted in terms of rotations of NbO(6) octahedra and displacements of the Sr atoms.