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Tuzlaite, NaCa[B5O8(OH)2 3H2O, is a monoclinic sheet borate [P21/c, a = 6.506(1), b = 13.280(3), c = 11.462(3) &#506 ; ; , &#946 ; ; = 92.97(2)°, V = 989(1) &#506 ; ; 3, Z = 4], occurring in nearly monomineralic veinlets formed in the course of dolomitic marl diagenesis at the Tuzla salt mine, located in the northeast part of Bosnia and Hercegovina. Intergrowths with halite have been observed where halite covers tuzlaite in later stages of diagenesis. The crystal structure, including H positions, was solved and refined from X-ray single- crystal data to R = 2.8, Rw = 3.3%. Tuzlaite possesses a new type of pentaborate sheet structure with fourfold- and threefold-coordinated B (5:2&#916 ; ; + 3T). Borate triangles and tetrahedra form ten-membered rings. The sheets are connected by Na and Ca, which are coordinated to additional H2O. The new mineral is colorless to white. It is biaxial positive, with the following refractive indices: nx = 1.532(2), ny = 1.544(2), and nz = 1.561(2). The optical orientation is Y = b, Z:a = 14° (in acute angle &#946 ; ; ). Tuzlaite has one perfect cleavage, parallel to {001}, and hardness 2-3 ; Dmeas = 2.21, Dcalc = 2.23 g/cm3. The chemical composition is CaO 14.64%, Na2O 10.25%, SrO 0.21%, B2O3 52.19%, Al2O3 0.26%, and H2O 21.66%. Tuzlaite is chemically related to the pentaborates probertite, NaCaB5O7(OH)4.3H2O, and ulexite, NaCaB5O6(OH)6.5H2O. The new mineral is named after the town Tuzla, where it was found.