Black shale-hosted vanadium (V) deposits account for about 80% vanadium resources in the world, but only B a 0.96 ⋅ H 2 O T i 5.87 V 1.87 3 + V 0.11 4 + S i 0.07 C r 0.07 F e 0.02 3 + O 16.00 , $ \left[\mathrm{Ba}_{0.96} \cdot \mathrm{H}_2 \mathrm{O}\right]\left(\mathrm{Ti}_{5.87} \mathrm{~V}_{1.87}^{3+} \mathrm{V}_{0.11}^{4+} \mathrm{Si}_{0.07} \mathrm{Cr}_{0.07} \mathrm{Fe}_{0.02}^{3+}\right) \mathrm{O}_{16.00},$ space group I41/a, unit-cell parameters a = b = 14.346(7) Å, c = 5.899(1) Å, α = β = γ = 90°, Z = 4. Data from EPMA, TESCAN integrated mineral analyzer (TIMA), and whole-rock geochemistry indicate that 12.32–44.06% (average 24.95%) vanadium exists in mannardite. Most vanadium atoms in mannardite occupy its structural sites as trivalent vanadium (V3+), forming chemical bonds with O atoms as V O 2 − , $\mathrm{VO}_2^{-},$ whereas a minor amount of vanadium atoms replace titanite atoms (Ti4+) as quadrivalent vanadium (V4+) by isomorphism. Mannardite precipitates under a strong reductive condition with sufficient trivalent vanadium species, titanium and biogenic barium (bio-barite). Our first identification of mannardite in black shale-hosted vanadium deposits thus sheds light on the occurrence mode of vanadium and the metallogenic mechanism of black shale-hosted vanadium deposits.