In the title compound, [Sm(C(48)H(45)N(10)O(3))(H(2)O)]·CH(3)CN, the Sm(III) ion is coordinated by the hepta-dentate tris-(2-{5-[(4-methyl-pheny)diazen-yl]-2-oxidobenzyl-idene-amino}-eth-yl)amine trianionic ligand and a water mol-ecule. The resulting SmN(4)O(4) coordination polyhedron is a distorted square anti-prism. In the crystal, complex mol-ecules are linked by O-H⋯O hydrogen bonds.