A rare supergene uranyl carbonate mineral, bayleyite, was found on a single specimen originated from the 3rd level of the Barbora shaft, the Jáchymov ore district, Czech Republic. Bayleyite occurs there as a yellow very small, imperfect crystals of glassy lustre forming crystalline aggregates up to 3 mm on a small fragment of the rock. Liebigite, schröckingerite, rabbittite and natrozippeite were found in the mineral association. According to single-crystal X-ray data bayleyite is monoclinic, space group P21/a, with a = 26.535(1), b = 15.244(2), c = 6.497(3) Å, β = 92.942(10), and V = 2624.5(13) Å3. Crystal structure was refined to R1 = 2.68% on the basis of 5008 unique observed reflections [Iobs.>3σ(I)] collected on a Oxford Diffraction Gemini diffractometer with a Atlas CCD detector and MoKα radiation. Chemical analyses of bayleyite yielded the average composition MgO 12.68, FeO 0.06, Al2O3 0.05, SiO2 0.45, UO3 51.97, CO2calc. 21.27, H2Ocalc. 29.46, total 115.94 wt. % corresponding to the empirical formula (Mg1.73 Al0.01Fe0.01)Σ1.75(UO2)1.00[(CO3)2.66(SiO4)0.04] •18H2O calculated on the basis of U = 1 apfu (CO3 content derived from the charge-balance). Raman spectrum of bayleyite is dominated by the symmetric stretching vibration of UO22+ ion, and stretching and bending modes of planar CO3 groups. Inferred uranyl bond-lenghts and possible hydrogen bond lenghts obtained from the Raman frequencies of corresponding vibrations, based on the empirical relations, are consistent with values from the refined structure.