Ronneburgite is a new chain vanadate mineral from Ronneburg, Thuringia, Germany. It occurs as reddish-brown crystals of equant, flattened, or short prismatic habit, up to 0.5 mm in length. The crystals have an adamantine luster and are translucent. Ronneburgite is biaxial negative with refractive indices nα = 1.925(5), nβ = 1.960(10), nγ = 1.988(4), and 2Vx = 82°. Electron microprobe analysis (K2O = 16.93, MnO = 12.44, MgO = 0.62, V2O5 = 68.54; total = 98.53 wt%) gives the empirical formula K1.91Mn0.93Mg0.08V4.00O11.96, which simplifies to K2MnV4O12. Ronneburgite is mono-clinic, space group P21/n, with a = 8.183(3), b = 9.247(3), c = 8.651(2) Å, and β = 109.74(2)°. The measured density is 2.84 g/cm3; the calculated density for Z = 2 is 2.85 g/cm3. Single-crystal X-ray structure determination (R1 = 0.046 for 1846 reflections) shows that ronneburgite contains infinite vierer-single metavanadate chains [(VO3)n]n− of corner-sharing VO4 tetrahedra along . Although the empirical formula and calculated bond valences indicate that V is 5+ and Mn is 2+, the crystal color, a “poor” structural compatibility index, and Mn-O distances between those expected for Mn2+-O and Mn3+-O indicate that some Mn3+ and V4+ ions are present. K+ ions and octahedral Mnn+ ions bridge between the metavanadate chains by coordinating to their oxygen atoms.