The title compound, C20H30O2 (tagalsin F), a diterpenoid, was isolated from the roots of Ceriops tagal (Perr.) collected from Nakhon Si Thammarat province, Thailand, as part of an investigation of bioactive compounds from mangroves. Its X-ray crystal structure was determined to establish relative stereochemistry, and it exhibited no antimalarial activity compared to artemisinin (IC50 3.3–3.9 nM). The molecule features a fused A/B/C tricyclic system with a cis A/B junction and trans B/C junction. Cyclohexane ring A adopts a half-chair conformation (puckering parameters Q = 0.464(2) Å, θ = 128.3(2)°, φ = 78.6(3)°), while rings B and C are in standard chair conformations. The hydroxymethylidene substituent attached to ring A forms a planar unit involving atoms O1, O2, C1, C2, C3, C4, and C18 (r.m.s. deviation 0.023(2) Å, C2—C1—C18—O1 torsion angle 0.4(2)°), stabilized by an intramolecular O—H…O hydrogen bond. Bond distances and angles are normal. Crystal packing is stabilized by weak intermolecular C—H…O interactions, with molecules arranged alternately in a head-to-tail and tail-to-head fashion along the b axis.