Hydroxylborite, a new mineral species, an analogue of fluoborite with OH > F, has been found at the Titovsky deposit (57°41′N, 125°22′E), the Chersky Range, Dogdo Basin, Sakha-Yakutia Republic, Russia. Prismatic crystals of the new mineral are dominated by the {10 [`1]\overline 1 0} faces without distinct end forms and reach (1−1.5) × (0.1−0.2) mm in size. Radial aggregates of such crystals occur in the mineralized marble adjacent to the boron ore (suanite-kotoite-ludwigite). Calcite, dolomite, Mg-rich ludwigite, kotoite, szaibelyite, clinohumite, magnetite, serpentine, and chlorite are associated minerals. Hydroxylborite is transparent colorless, with a white streak and vitreous luster. The new mineral is brittle. The Mohs’ hardness is 3.5. The cleavage is imperfect on {0001}. The density measured with equilibration in heavy liquids is 2.89(1) g/cm<sup>3</sup>; the calculated density is 2.872 g/cm<sup>3</sup>. The wave numbers of the absorption bands in the IR spectrum of hydroxylborite are (cm<sup>−1</sup>; sh is shoulder): 3668, 1233, 824, 742, 630sh, 555sh, 450sh, and 407. The new mineral is optically uniaxial, negative, ω = 1.566(1), and ε = 1.531(1). The chemical composition (electron microprobe, H<sub>2</sub>O measured with the Penfield method, wt %) is 18.43 B<sub>2</sub>O<sub>3</sub>, 65.71 MgO, 10.23 F, 9.73 H<sub>2</sub>O, 4.31-O = F<sub>2</sub>, where the total is 99.79. The empirical formula calculated on the basis of 6 anions pfu is as follows: Mg<sub>3.03</sub>B<sub>0.98</sub>[(OH)<sub>2.00</sub>F<sub>1.00</sub>]O<sub>3.00</sub>. Hydroxylborite is hexagonal, and the space group is P6<sub>3</sub>/m. The unit-cell dimensions are: a = 8.912(8) Å, c = 3.112(4) Å, V = 214.05(26) Å<sup>3</sup>, and Z = 2. The strongest reflections in the X-ray powder pattern [d, Å (I, %)(hkil)] are: 7.69(52)(01 [`1]\overline 1 0), 4.45(82)(11 [`2]\overline 2 0), 2.573(65)(03 [`3]\overline 3 0), 2.422(100)(02 [`2]\overline 2 1), and 2.128(60)(12 [`3]\overline 3 1). The compatibility index 1 − (K _p/K _c) is 0.038 (excellent) for the calculated density and 0.044 (good) for the measured density. The type material of hydroxylborite is deposited in the Fersman Mineralogical Museum, Russian Academy of Sciences, Moscow (inventory number 91968) and the Geological Museum of the All-Russia Institute of Mineral Resources, Moscow (inventory number M-1663).