Jadarite, ideally LiNaSiB3O7(OH), is a new mineral species from the Jadar Basin, Serbia. It occurs as massive white aggregates, several metres thick, and is relatively free from inclusions and intergrowths; however, individual subhedral (tabular, elongate) to anhedral crystals rarely exceed 5–10 μm in size. It is associated with calcite, dolomite, K-feldspar, rutile, albite, ilmenite, pyrite, and fine-grained muscovite. Searlesite, analcime, chlorite, and quartz have also been identified. Jadarite is translucent (opaque in masses) with a porcellanous lustre (masses), possesses a white streak, is brittle with a platy habit and has an uneven to conchoidal fracture. VHN200 is 390 (range 343–426) kg/mm2. Mohs’ hardness is 4–5. It shows weak pink-orange fluorescence under both short- and long-wave ultraviolet radiation. An infra-red adsorption spectrum is given and shows strong, sharp peaks at 3490 and 3418 cm−1 which indicates that water is present as (OH) only. Peaks at 1409 and 1335 cm−1 indicate the presence of BO3 groups, and between 900 and 1180 cm−1 the probable presence of BO4. In transmitted light, plates and grains of jadarite show twinning in some crystallites and for λ 590 nm nα = 1.536(± 0.001) and nγ = 1.563(± 0.001). It is non-pleochroic, biaxial, and does not show parallel extinction. In plane-polarized reflected light, the mineral is dark grey with weak bireflectance, it is non-pleochroic and has abundant white internal reflections. Wet chemical analysis combined with CHN analyzer gave the following aggregate composition: Li2O 7.3, Na2O 15.0, SiO2 26.4, B2O3 47.2, H2O 4.3, total 100.2 wt.%. The empirical formula, based on 3B atoms per formula unit (apfu), is: Li1.08Na1.07Si0.97B3O6.99(OH)1.06. Jadarite is monoclinic (P21/n) with a 6.818(2), b 13.794(2), c 6.756(2) Å, β 111.10(2)° V 592.8(2) Å3 (Z = 4), alternatively (P21/c) with a 6.756(3), b 13.794(2), c 7.680(3) Å, β 124.07(3) °, V 592.9(4) Å3 and Z = 4. The measured density (Berman Balance) is 2.45 g/cm3; calculated density is 2.46 g/cm3 (on the basis of the empirical formula and unit-cell parameters refined from powder data). The six strongest X-ray powder-diffraction lines [d̅ in Å(I)(hkl)] are: 4.666 (62) (120, 021), 3.180 (82) (200), 3.152 (74) (002), 3.027 (40) (221), 2.946 (100) (131), 2.241 (74) (3 11,151), The mineral name is for the locality in Serbia where it was discovered during mineral exploration of the Jadar Basin.