Clinometaborite, β-HBO2, the monoclinic modification of metaboric acid, which has long been known as a synthetic product only, was found as a sublimate in an active medium-temperature intracrater fumarole at La Fossa crater, Vulcano Island, Aeolian archipelago, Sicily, Italy. It forms aggregates of stout crystals up to 2 mm long in an altered pyroclastic breccia, associated with metaborite, sassolite and adranosite. The unaltered mineral is colorless and translucent, with a vitreous luster; it becomes chalky white after some months of exposure to open air. The crystal habit is prismatic; twinning was not observed. The strongest six lines in the X-ray powder-diffraction pattern [dobs in Å(I)hkl] are: 3.078(100)201, 4.193(20)111, 6.773(15)001, 2.550(10)1̄22, 3.224(8)2̄01, and 2.702(8)031. The unit cell is monoclinic, space group P21/a, with a 7.127(2), b 8.842(3), c 6.773(2) Å, β 93.21(1)°, V 426.1(2) Å3, Z = 12, values that virtually coincide with those of the synthetic compound. The calculated density is 2.049 g/cm3. The structure was refined to a final R of 0.043 for 977 observed reflections [I > 2σ(I)]. Both the mineral and the mineral name have been approved by the IMA Commission on New Minerals, Nomenclature and Classification (IMA 2010–022).