Abstract: Protasite, a new barium-containing member of the uranyl oxide hydrate group, occurs as bright orange pseudo-hexagonal platelets associated with uraninite and uranophane on unidentified rock matrix from Shinkolobwe mine, Zaïre. Protasite is monoclinic, Pn with a 12.295(2), b 7.221(1), c 6.9558(8) Å, β 90.40(2)°, and V 617.50(11) ų. The tabular pseudo-hexagonal crystals are flattened on {010}, 0.1 mm to 0.5 mm wide and up to 0.1 mm thick. They are biaxial negative, 2 V = 60–65°, β and γ 1.79–1.83, and X = b. Sector twinning is common. Microprobe analysis shows BaO 15.0, UO₃ 78.0, H₂O(diff.) 7.0%. The structural formula is Ba[(UO₂)₃O₃(OH)₂]. 3H₂O, Z = 2, and density(calc.) = 5.827(3) g cm⁻³. Complete crystal-structure analysis shows protasite to be the simplest model structure of the uranyl oxide hydrate group.