This paper documents the occurrence of tinsleyite in the “dry” Cioclovina Cave, Sureanu Mountains, Romania. This is the first mention of tinsleyite as a cave mineral and the second find of this mineral species in the world. The mineral occurs in small quantities, as a rare authigenic, early diagenetic mineral, in the bat guano deposits in this cave. Low (alpha) quartz is the only closely associated mineral. This assemblage corresponds to a possible reaction in the natural guano - clay system upon decomposition of illite. Chemical analyses show variable leucophosphite contents (up to 31.68 mol.%). The unit-cell parameters calculated for an iron-poor sample are a = 9.638(5) Å, b = 9.522(4) Å, c = 9.540(4) Å and β = 103.10(3)°, whereas those determined for an iron-rich sample are a = 9.639(6) Å, b = 9.538(8) Å, c = 9.553(7) Å and β = 103.04(4)°. Thermal analyses show that water is lost in three steps and that the loss of molecular water is complete before 250°C. The fully dehydrated material is completely X-ray amorphous. Ammonium is expelled at about 490°C. The infrared spectrum of tinsleyite affords reasonable evidence for the presence of PO4, NH4 and OH groups, together with H2O molecules. The punctual symmetry of the PO4 groups is Cs.