Single-crystals of thallium(I) iodide oxide Tl3IO were obtained as by-product in a hydro­flux synthesis at 473 K for 10 h. A potassium hydroxide hydro­flux with a water-base molar ratio of 1.6 and the starting materials TlNO3, RhI3 and Ba(NO3)2 was used, resulting in a few black needle-shaped crystals. X-ray diffraction on a single-crystal revealed the hexa­gonal space group P63/mmc (No. 194) with lattice parameters a = 7.1512 (3) Å and c = 6.3639 (3) Å. Tl3IO crystallizes as hexa­gonal anti-perovskite (anti-BaNiO3 type) and is thus structurally related to the alkali-metal halide/auride oxides M3XO (M = K, Rb, Cs; X = Cl, Br, I, Au). The oxygen atoms center thallium octa­hedra. The [OTl6] octa­hedra share trans faces, forming a linear chain along [001]. Twelve thallium atoms surround each iodine atom in an [ITl12] anti-cubocta­hedron. Thallium and iodine atoms together form a hexa­gonal close-sphere packing, in which every fourth octa­hedral void is occupied by oxygen.