TlFeSe2 crystallizes in the tetragonal I-4m2 space group. The structure is three-dimensional. Fe3+ is bonded to four equivalent Se2- atoms to form corner-sharing FeSe4 tetrahedra. All Fe–Se bond lengths are 2.38 Å. Tl1+ is bonded in a body-centered cubic geometry to eight equivalent Se2- atoms. All Tl–Se bond lengths are 3.40 Å. Se2- is bonded in a distorted water-like geometry to two equivalent Fe3+ and four equivalent Tl1+ atoms.