Watatsumiite, KNa2LiMn2V2Si8O24, the V- and Mn-analogue of neptunite, is found in metamorphosed manganese ore deposit of the Tanohata mine, Iwate Prefecture, Japan. The mineral is monoclinic, Cc, a = 16.450(16) A, b = 12.492(7) A, c = 9.995(8) A, β = 115.32(6)°, V = 1857(2) A3 and Z = 4. The seven strongest lines in the X-ray powder diffraction pattern are [d (A), (I), (hkl)]: 9.58(84)(110), 4.52(85)(002), 3.52(63)(131), 3.19(100)(330, 132), 2.94(90) (223), 2.90(66)(222, 510) and 2.49(93)(441, 621). Electron microprobe analysis and laser ablation microprobe-inductively coupled plasma-mass spectrometry gave SiO2 52.64, TiO2 3.13, VO2 15.10, FeO 0.35, MnO 12.28, MgO 1.61, CaO 0.03, BaO 0.88, Na2O 7.10, K2O 4.89, Li2O 1.6, total 99.61 wt.%, corresponding to (K0.94Ba0.05)Σ0.99Na2.08Li0.97(Mn1.57Mg0.36Fe0.04)Σ1.97(V1.66Ti0.36)Σ2.02Si7.97O24 on the basis of O = 24. Watatsumiite is transparent and yellowish green with vitreous luster. The streak is white with greenish tint. The cleavage is poor. The fracture is conchoidal and brittle. The hardness is VHN100 707-946 kg/mm2 (Mohs 51/2-6). The calculated density is 3.24 g/cm3. Watatsumiite has moderate pleochroism from yellowish tint to pale yellowish green, and is optically biaxial positive with 2V = 60(5)°, α = 1.686 (2), β (calc) = 1.694, γ = 1.720 (5), v > r perceptible. Watatsumiite occurs as short prismatic or massive aggregates in veinlets composed mainly of quartz, K-feldspar and potassicleakeite.