Electron-microprobe analyses of hydroxylellestadite from the Cioclovina Cave (Romania) gave the composition Ca10.27[(SiO4)2.53(SO4)2.17(PO4)1.27]∑=5.97[(OH)1.66F0.21Cl0.16]∑=2.03. The mineral is translucent to transparent, light orange, slightly fluorescent, has a vitreous luster and <1.5 mm in length. A single-crystal X-ray structure investigation gave the average space-group symmetry P63/m [R1(F) = 0.038 for 783 reflections up to 2𝛉MoKα = 70° and 42 variables, a = 9.496(2), c = 6.920(2) Å, V = 540.4 Å3, and Z = 2]. Some atoms exhibit large anisotropic displacements. Ordering of atoms along with a symmetry reduction is not verified. Fourier-transformed infrared (FT-IR) and micro-Raman spectra exhibit a distinct contribution from (PO4)3− modes along with the characteristic (SO4)2− and (SiO4)4− modes. The occurrence is quite unusual and suggests that an intense thermal process affected a restricted area within the cave. Hydroxylellestadite is associated with berlinite, another high-temperature mineral. It is likely to have formed within highly phosphatized, silicate-rich, carbonate-mudstone sediments heavily compacted and thermally transformed due to in situ bat guano combustion.