An incommensurately modulated structure of CeSi_1.82 as determined by single-crystal X-ray diffraction at room temperature is reported. The observed satellite reflections reduce the orthorhombic symmetry of the X-ray pattern to a monoclinic one. This suggests the description of the modulated structure in the monoclinic superspace group I2/b(aβ0)00. The modulation wave vector was determined to q^→ = 0.410(1) · a ^→* + 0.610(1) · b^→*. The refinement was done using harmonic modulation functions for the displacement and the occupation probability of the atoms, the quality parameters converged to R_obs = 3.33% (R_obs = 2.56% for the main reflections, R_obs = 8.88% for the 1st order satellite reflections and R _obs = 33.06% for the 2nd order satellite reflections). The origin of the observed satellite reflections is attributed to a modulation of the partly occupied Si site caused by composition changes within the crystal.