Bariosincosite is a new barium vanadium phosphate hydrate from the Spring Creek Mine, near Wilmington, South Australia. The new mineral occurs as irregular clusters of pale green, very thin platey crystals up to 250 µm across and 2 to 5 µm thick. The tetragonal crystals are tabular on {001} and the other form present is {100}. Associated with bariosincosite are quartz, cuprite, native copper, fluorapatite, whitlockite, baryte and springcreekite, BaV₃³⁺(PO₄)₂(OH,H₂O)₆. Bariosincosite appears to have formed under supergene or low-temperature late-stage hydrothermal conditions. Electron microprobe analysis yielded: BaO 23.20; SrO 4.19; CaO 0.36; VO₂ 31.55; Fe₂O₃ 0.20; Al₂O₃ 0.50; P₂O₅ 28.15; H₂O 13.93 (calculated). These data give an empirical formula of (Ba_0.77Sr_0.20Ca_0.03)_Σ1.00[(V_0.96⁴⁺Al_0.03Fe_0.01³⁺)_Σ1.00O(PO₄)]₂·4H₂O, calculated on the basis of two P atoms. The simplified formula is Ba(V⁴⁺OPO₄)₂·4H₂O. The mineral is transparent with a very pale green streak, a vitreous lustre and an estimated Mohs hardness of 3. The strongest lines in the X-ray powder pattern are [d_obs (I_obs) (hkl)] 6.414 (20) (110, 002); 5.748 (70) (111); 4.552 (30) (112, 200); 3.198 (20) (220, 004); 3.100 (100) (203, 221); 2.847 (40) (222, 114); 2.786 (80) (311); 2.368 (30) (313, 115); and 2.017 (100) (420, 332, 116). These data were indexed on a tetragonal cell, with a = 9.031(6), c = 12.755(8) Å and V = 1040(1) ų; the space group is probably P4/n or P4/nmm. For Z = 4 and using the empirical formula, the calculated density is 3.306 gm/cm³. Bariosincosite is uniaxial negative with ω = 1.721(2) and ε = 1.715(2) (white light); pleochroism is weak from colourless (E) to pale green (O), absorption O > E. The mineral is named for the relationship to sincosite, Ca(V⁴⁺OPO₄)₂·4H₂O.