The structure of naturally-occurring cinerin C [systematic name: (7S,8R,3'R,4'S,5'R)-Δ(8')-4'-hydroxy-5,5',3'-trimethoxy-3,4-methylenedioxy-2',3',4',5'-tetrahydro-2'-oxo-7.3',8.5'-neolignan], isolated from the ethanol extract of leaves of Pleurothyrium cinereum (Lauraceae), has previously been established by NMR and HRMS spectroscopy, and its absolute configuration established by circular dichroism measurements. For the first time, its crystal structure has now been established by single-crystal X-ray analysis, as the monohydrate, C(22)H(26)O(7)·H(2)O. The bicyclooctane moiety comprises fused cyclopentane and cyclohexenone rings which are almost coplanar. An intermolecular O-H···O hydrogen bond links the 4'-OH and 5'-OCH(3) groups along the c axis.