We prepared a three-dimensional monometallic lanthanoid assembly, Na(5)[Ho(THB(4-))(2)].7H(2)O (THB = 1,2,4,5-tetrahydroxybenzene), that exhibits ferromagnetism with a Curie temperature of 11 K. Such a ferromagnetic ordering is due to the effective mediation of the magnetic interaction between Ho(3+) ions by the THB(4-) ligand.