Bluestreakite, K4Mg2(V4+2V5+8O28)·14H2O, is a new mineral species from the Blue Streak mine, Bull Canyon, Montrose County, Colorado, USA. Bluestreakite typically occurs as irregular polycrystalline coatings on rounded quartz grains or masses of montroseite, and rarely as tablets or blades. It grows on corvusite-montroseite-bearing sandstone blocks intimately associated with gypsum, huemulite, hummerite, metamunirite, and munirite. Bluestreakite is dark greenish blue, with a light blue streak. The mineral is transparent, with a subadamantine luster. Bluestreakite does not fluoresce in short- or long-wave ultraviolet radiation, and it has a hardness ca. 2. Bluestreakite has a brittle tenacity, irregular fracture, and no cleavage. Density (calc.) = 2.630 g⋅cm−3 based on the empirical formula and single-crystal cell data. Bluestreakite is biaxial (–), with α 1.750(5), β 1.800(5), γ 1.829 (calc.) (white light); 2Vmeas. = 73(3)°. Optic orientation is Z = b (X and Y not determined). The pleochroism is slight, with X < Y ≈ Z dark greenish blue. Electron probe microanalysis gave the empirical formula (based on O = 42) (K3.37Na0.15)Σ3.52Mg1.94(V4+1.40V5+8.60)O28·14H2O. Bluestreakite is monoclinic, P21/n, with a 12.2383(7), b 10.3834(4), c 14.1945(6) Å, β 103.008(2)°, V 1757.48(14) Å3, and Z = 2. The strongest four lines in the diffraction pattern are [d in Å(I)(hkl)]: 10.34(57)(01), 8.27(100)(011,101), 7.90(21)(110), and 1.9814(22)(17,17). The atomic arrangement of bluestreakite was solved and refined to R1 = 0.0339 for 7395 independent reflections with F > 4σ (F). The structural unit in bluestreakite is a partially reduced decavanadate group, with an ideal composition of [(V4+2V5+8)O28]8−; charge balance in the structure is maintained by the [K4Mg2⋅14H2O]8+ interstitial complex. The interstitial complex is formed of irregular K1[O4(H2O)4] and K2[O6(H2O)3] polyhedra, and a Mg(H2O)6 octahedron. Bonding between the interstitial complex and the structural unit takes place through direct bonding of the K1 and K2 atoms of the interstitial complex with the oxygen atoms of the structural unit, as well as extensive hydrogen bonding.