The synthesis, and crystal and molecular structure of N,N'-diphenylguanidinium m-chlorobenzeneseleninate (C19H18ClN3O2Se), M(r) 434.78, are reported. The crystal lattice of the compound belongs to the monoclinic space group P2(1)/c with a 7.277(1), b 12-793(2), c 20.013(4) angstrom, beta-94.733(12)-degrees, V 1859.8 angstrom3, Z 4, D(obs) 1. 53 (by flotation), D(calc) 1.555 g cm-3, lambda(Mo K-alpha) 0.71069 angstrom, mu-21.0 cm-1, F(000) 880, T 293 K, R 0.057 and R(w) 0.059 for 1745 unique observed reflections with I > 2.0-sigma(I). The crystal packing consists of parallel layers of anions intercalated with cations interacting through electrostatic contacts. Vibrational spectra have been also recorded, and are discussed.