Both borates and sulfides are important inorganic multifunctional materials. Encouraged by this background, thioborates attract considerable interest. However, their investigations are highly hindered by the scarcity of the available ones and the synthetic difficulty of the new ones. Here, we report a new thioborate KEu2In3B12S13 (1), which was obtained via a facile solid-state reaction in KI flux. It crystallizes in the trigonal R3̄m structure, and the three-dimensional structure features a six InS6 octahedron consolidated B12 icosahedron built {[In3B12S12]5-}∞ polyanionic framework and a unique In6S6 12-membered ring, representing a new type of compound. The B12S12 cluster is also different from the other known thioborates. The structural chemistry, optical and magnetic properties, as well as theoretical calculations of 1 were systematically studied. This study not only provides a new type of thioborate but also makes a breakthrough in the synthesis of thioborates.