Rameauite is a rare supergene uranyl-oxide hydroxy-hydrate mineral that forms during hydration-oxidation weathering of uraninite. On the basis of single-crystal X-ray diffraction data collected on a microfocus source, rameauite is monoclinic, space group Cc, with a = 13.9458(19), b = 14.3105(19), c = 13.8959(18) Å, β = 118.477(14)°, V = 2437.7(6) Å3 and Z = 4, with Dcalc = 5.467 g cm−3. The structure of rameauite (R = 0.060 for 1698 unique observed reflections) contains sheets of the β-U3O8 topology, with both UO6 and UO7 bipyramids, which is similar to the sheets found in spriggite, ianthinite and wyartite. The sheets alternate with the interlayer, which contains K+, Ca2+ and H2O molecules. Interstitial cations are linked into infinite chains that extend along [10-1]. Adjacent sheets are linked through K–O, Ca–O and H-bonds. The structural formula of rameauite is K2Ca(H2O)1(H2O)4[(UO2)6O6(OH)4](H2O)1. The empirical formula obtained from the average of eight electron-microprobe analyses is (on the basis of 6 U p.f.u.) K1.87(Ca1.10Sr0.04)Σ1.14[(UO2)6O6(OH)4.15]·6H2O. The Raman spectrum is dominated by U–O and O–H vibrations. A discussion of related uranyl-oxide minerals is given.