Abstract: Parasibirskite, with the ideal formula Ca₂B₂O₅·H₂O, is a new mineral species found at Fuka, Okayama Prefecture, Japan. It is a polymorph of sibirskite, CaHBO₃, and occurs as subparallel aggregates of tabular crystals up to 40 × 20 × 3 µm in size. Associated minerals are takedaite, olshanskyite, sibirskite, frolovite and calcite. The mineral is white, and has a weak pearly luster. Optically, the mineral is biaxial positive, α 1.556(2), β 1.593(2), γ 1.663(2) (λ, 589 nm). The Vickers microhardness of aggregates is 121 kg mm⁻². The mineral is monoclinic with space group of P2₁/m, a 6.722(4), b 5.437(2), c 3.555(2) Å, β 93.00(5)°, V 129.8(2), ų. The strongest lines in the X-ray powder pattern [d in Å (I)(hkl)] are 2.237(100)(300), 6.73(70)(100), 2.975(60)(011), 3.354(30)(200), 2.855(20)(210) and 1.776(20) (002). Wet chemical analysis, electron-microprobe analysis and ICP emission spectrometry give the values CaO 56.06 %, B₂O₃ 34.10 %, H₂O 9.97 % and total 100.13%. The empirical formula calculated on the basis of O = 6 is Ca_1.985B_1.945O_4.901·1.099H₂O, for Z = 1, D_calc 2.54 and D_meas 2.50(1) g cm⁻³. Parasibirskite is formed by hydrothermal alteration of takedaite.