The title compound with heteratomic anionic chains (1)(infinity)[Tl4Sb612-] has been discovered in the K-Tl-Sb system. The phase is obtained from a range of compositions near K3TlSb1.5 following reaction first at 750-850 degreesC and then at 550 degreesC for one week or more. It crystallizes in the monoclinic system in space group C2/c, Z = 8, a = 9.951(1) Angstrom, b = 17.137(3) Angstrom, c = 19.640(6) Angstrom, and beta = 104.26(3)degrees Swing-like (Tl4Sb6)(12-) units consisting of alternating Sb and Tl atoms in four- and eight-membered rings are linked through Tl-Tl bonds to form infinite one-dimensional chains along (a) over bar. EHTB calculations and resistivity measurements show that the compound is a semiconductor.