Neutron and x-ray powder and single crystal synchrotron diffraction of Cs_yFe_2-xSe₂ show the presence of superstructure reflections with propagation vector k=[²/₅,¹/₅,1] with respect to the average crystal structure I4/mmm (a ∼ 4, c ∼ 15 Å). The propagation vector star corresponds to the 5 times bigger unit cell given by transformation A = 2a+b, B = -a+2b, C=c. A solution for the atomic structure is found in the space group I4/m with an ordered pattern of iron vacancies corresponding to the iron deficiency x = 0.29 and Cs stoichiometry y = 0.83. The superstructure satellites are more pronounced in the neutron diffraction patterns suggesting that they can have some magnetic contribution. We have sorted out all possible symmetry adapted magnetic configurations and found that the presence of antiferromagnetic ordering with the ordered magnetic moment of Fe with ≃2μ_B does not contradict the experimental data. However, the solutions space is highly degenerate and we cannot choose a specific solution. Instead we propose possible magnetic configurations with the Fe magnetic moments in (ab) plane or along c axis. The superstructure is destroyed above T_s ≃ 500 K by a first-order-like transition.