The crystal structure of synthetic Tl feldspar, TIAISi3O8, was determined based on single-crystal X-ray diffraction data; monoclinic, a = 8.882(3), b = 13.048(2), c = 7.202(2) Å, β = 116.88(1)°, V = 744.5(4)Å3, Z = 4, space group C2/m (R = 7.33% for 462 observed reflections). The underlying framework is similar to Rb feldspar, RbAlSi3O8, and can be regarded as isotypic with sanidine. The structure accommodates Tl+ cations occupying the M site coordinated by nine O atoms. The more expanded Tl polyhedra as compared to the Rb polyhedra, in spite of the small ion radius of Tl+ relative to Rb+, result from the behavior of the stereoactive lone-pair electrons of Tl+ called “inert-pair effect”. The difference in centroid-central atom distance in the TlO9 polyhedron suggests that the lone-pair electrons are orienting parallel to the  direction in the feldspar structure. Therefore, Tl found as a trace element in analyses of natural K-feldspar can be considered as an actual constituent of this mineral. The syntheses of the present Tl-feldspar, TlAlSi3O8, together with Tl-leucite, TlAlSi2O6, and the chemical analogue of kalsilite, TlAlSiO4, shed new light on the crystal chemistry of potassium aluminosilicate minerals.