The new ettringite-group mineral chiyokoite, ideally Ca₃Si(CO₃)[B(OH)₄]O(OH)_5'12H₂O, was found in a hydrothermally altered calc-silicate skarn at the Fuka mine, Okayama Prefecture, Japan. Associated minerals are calcite, henmilite, and tacharanite. Chiyokoite occurs as hexagonal prismatic crystals up to 30 lm long and up to 20 lm thick. The major forms are the hexagonal prism {10-10} and monohedra {0001} and {000-1}. The crystals are combined in clusters which form friable nests up to 1 cm across. The mineral is pink to colorless with white streak and vitreous luster. The cleavage is parallel to {10-10} and {0001}, good. The fracture is stepped. D_meas is 1.85(1) g/cm³, D_calc is 1.85 g/cm³. Chiyokoite is optically uniaxial (-), x ¼ 1.523(2) and e ¼ 1.492(3) (589 nm). The infrared spectrum is reported. The chemical composition (wt.%) is CaO 27.56, B₂O₃ 3.47, Al₂O₃ 3.05, Fe₂O₃ 0.12, As₂O₃ 4.77, MnO₂ 0.32, SiO₂ 6.55, SO₃ 0.76, H₂O 46.3, CO₂ 7.30, total 100.2. The empirical formula calculated on the basis of 3 Ca apfu is H_31.37Ca₃(Si_0.67Al_0.37Mn^4þ_0.02Fe^3þ_0.01)_R1.07(C_1.01B_0.61As^3þ_0.29S_0.06)_R1.97O_24.19. The simplified general formula is Ca₃(Si,Al)(CO₃,AsO₃)[B(OH)₄,AsO₃](OH)_6'12H₂O. Chiyokoite is hexagonal, P6₃, a ¼ 11.0119(5), c ¼ 10.5252(6) Å, and V ¼ 1105.3(1) ų. The strongest reflections of the powder X-ray diffraction pattern [d,Å(I)(hkl)] are: 9.53(100)(100), 5.50(24)(110), 4.618(11)(102), 3.812(23)(112), 3.412(15)(211), 2.726(14)(302), 2.521(19)(123), and 2.172(13)(320,402,223). The crystal structure, refined from single-crystal X-ray diffraction data [R1(F) ¼ 0.042], is based on [Ca₃(Si,Al)(OH)₆(H₂O)₁₂] columns parallel to the c axis with B(OH)₄^- and CO₃^2- and admixed AsO₃^3-anionic groups in channels between the columns. The mineral is named in honor of Professor Chiyoko Henmi (1949-2018).