A new species of the thermophilous Tetramorium caespitum species complex, T. sibiricum n. sp., is described from the Central Siberian region near Ulan Ude that has mean January temperatures of – 24 °C. The new species is clearly separable from the related species, T. indocile Santschi, 1927 and T. caespitum (Linnaeus, 1758), by exploratory data analyses of 35 phenotypic characters and by a discriminant analysis of seven phenotypic characters. A key to these three species, which all might occur in Central Siberia, is provided. The zoogeographic divide called the Reinig Line (De Lattin, 1967) is considered to be important in separating the ranges of Central and East Palaearctic ant species with less strong cold-hardiness. Based on images of type specimens, Tetramorium annectens Pisarski, 1969 is recognized as heterospecific from T. tsushimae Emery, 1925.