The compound Th2Se5 has been synthesized and its structure determined by means of single-crystal X-ray diffraction methods. The subcell of Th2Se5 is in the tetragonal space group P4(2)/nmc and is isostructural to Np2Se5. The modulated structure of Th2Se5 has been solved in the monoclinic super space group P2(alpha 0 gamma)0. The structure features parallel infinite Se-Se chains with minimum and maximum Se-Se distances of 2.477(5) and 2.967(5) angstrom, respectively. Th2Se5 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 Th2Se5 is 0.37(2) eV. That the Th2Se5 structure is modulated whereas the Np2Se5 structure appears not to be implies that the formal oxidation state of Np in Np2Se5 is 4+.