The compound Th(2)Se(5) has been synthesized and its structure determined by means of single-crystal X-ray diffraction methods. The subcell of Th(2)Se(5) is in the tetragonal space group P4(2)/nmc and is isostructural to Np(2)Se(5). The modulated structure of Th(2)Se(5) has been solved in the monoclinic super space group P2(α0γ)0. The structure features parallel infinite Se-Se chains with minimum and maximum Se-Se distances of 2.477(5) and 2.967(5) Å, respectively. Th(2)Se(5) is a semiconductor with an electrical resistivity that shows thermally activated Arrhenius behavior with an activation energy of 0.40(1) eV. From spectroscopic measurements, the band gap of Th(2)Se(5) is 0.37(2) eV. That the Th(2)Se(5) structure is modulated whereas the Np(2)Se(5) structure appears not to be implies that the formal oxidation state of Np in Np(2)Se(5) is 4+.