Trisilver(I) amidoselenate-3-ammonia-2-water, Ag3SeO3N•3NH3•2H2O, Mr = 551.70, is orthorhombic, space group P212121 with a = 11.864(3), b = 14.550(3), c = 6.433(2) Å, Z = 4, Dm = 3.25, Dx = 3.30 Mg m−3, radiation [Formula: see text], λ = 0.71069 Å, μ = 8.62 mm−1, F(000) = 1024, T = 295 K. The structure was solved by direct methods and refined to R = 0.036 for 1759 observed reflections. In the structure, only two independent Ag atoms are directly bonded to the N(Se) atom of the amidoselenic group (NSeO3) and these groups are joined by Ag into an infinite polymeric chain via [—N(Se)—Ag] bridges. The third Ag atom is present as a complex cation [Ag(NH3)2]+. Each silver atom is coordinated by two N atoms: Ag(1) by N(Se) and a NH3 molecule, Ag(2) by two N(Se) atoms, and Ag(3) by two NH3 molecules. Important blood lengths are Ag(1)—N(Se), NH3 2.111 and 2.110(8); Ag(2)—N(Se), N(Sei) 2.106 and 2.096(7); and Ag(3)—NH3, NH3 2.115 and 2.150(10); N(Se)—Se 1.727(8); and Se—O 1.641, 1.647, and 1.639(7) Å. Key words: Silver amidoselenate, silver–nitrogen bond, nitrogen–selenium bond.