In the title compound, C₂₄H₂₉ClO₈·2.5H₂O, which contains two organic mol­ecules (A and B) and five heavily disordered water mol­ecules in the asymmetric unit, the γ-lactone ring and the cyclo­hexenone ring are both trans-fused to the central cyclo­decene ring. The cyclehexenone ring features an α,β-unsaturated ketone with torsion angles between the conjugated carbonyl and alkene bonds of 0.6 (3) and 7.4 (4)° for mol­ecules A and B, respectively. The ptilosarcenone torsion angles between conjugated alkene bonds are 56.2 (5) and 55.4 (6)° for A and B, respectively. In the crystal, the components are linked by O—HO hydrogen bonds. The absolute configuration of ptilosarcenone was determined unambiguously and exhibits similar absolute stereochemistry to that found in the crystal structures of other octocoralline briaranes.