A new trigonal (3T) polytype of chloritoid, Fe2+ Al2(SiO4)O(OH)2, a rock-forming schist mineral, has been found in the samples from the chloritoid type locality, Kosoy Brod deposit, Middle Urals, Russia. The single-crystal X-ray diffraction indicated that the mineral crystallizes in the space group P32, a = 5.4890(2), c = 26.7612(14) Å, V = 698.27(6) Å3. The structure was solved and refined to R1 = 0.0564 (wR2 = 0.1545) for 2534 unique reflections with I > 2σ(I). After known triclinic (1A) and monoclinic (2M2) modifications, chloritoid-3T is the third chloritoid polytype. The crystal structures of all three polytypes are based upon the same types of octahedral and tetrahedral layers. The main difference between the polytypes is in the stacking of dioctahedral layers L1 and trioctahedral layers L2 interconnected with tetrahedral net. The new polytype (3T) can be detected by its powder X-ray diffraction pattern. The complexity analysis indicates that the crystal structure of chloritoid-3T is the most complex among known polytypes. The discovery of a new chloritoid polytype is essential for the potential development of chloritoid-based geothermometers and the general conception of polytypism