Using surface x-ray diffraction and scanning tunneling microscopy in combination with first-principles calculations, we have studied the geometric and electronic structure of Cs-deposited Bi₂Se₃(0001) surface kept at room temperature. Two samples were investigated: a single Bi₂Se₃ crystal, whose surface was Ar sputtered and then annealed at  ∼ 500 ^°C for several minutes prior to Cs deposition, and a 13-nm-thick epitaxial Bi₂Se₃ film that was not subject to sputtering and was annealed only at  ∼ 350 ^°C. In the first case, a considerable fraction of Cs atoms occupy top layer Se atoms sites both on the terraces and along the upper step edges where they form one-dimensional-like structures parallel to the step. In the second case, Cs atoms occupy the f cc hollow site positions. First-principles calculations reveal that Cs atoms prefer to occupy Se positions on the Bi₂Se₃(0001) surface only if vacancies are present, which might be created during the crystal growth or during the surface preparation process. Otherwise, Cs atoms prefer to be located in f cc hollow sites in agreement with the experimental finding for the MBE-grown sample.