Caribbean gorgonians of the genus Pseudopterogorgia are known to produce diverse chemically interesting and biologically significant secondary metabolites. Recently, we examined the constituents of Puerto Rican specimens of Pseudopterogorgia acerosa and report the isolation and structure determination of alanolide (3), a novel tetracyclic norditerpene biogenetically related to tobagolide. The gorgonian was collected by SCUBA at La Parguera, Puerto Rico, and extracted with CHCl3/CH3OH (1:1). Purification via silica gel column chromatography and normal-phase HPLC afforded alanolide (3) as a colorless oil (yield 0.0002% wet weight). Its structure, including relative stereochemistry of seven chiral centers, was unequivocally established using 1H/13C NMR (COSY, long-range COSY, NOESY, HMQC, HMBC) and high-resolution mass spectrometry (HREIMS, HRFABMS). Alanolide is the first norpseudopterane natural product, featuring a unique carbon framework lacking the methyl (or carbomethoxyl) group normally at C4 in regular pseudopteranes, an unusual C4,12 oxabridge, and a unique spiro α-epoxy γ-lactone moiety. Biologically, neither pseudopterolide (1) nor alanolide (3) showed cytotoxicity against HCT 116, CCRF-CEM, or MCF-7 human tumor cell lines at 50 µg/mL, and pseudopterolide was inactive against HIV in the NCI test.