Palytoxin is one of the most potent and complex marine toxins, whose biogenetic origin was questioned due to marked seasonal and regional variations and sporadic occurrence in alga, crabs, and a herbivorous fish, suggesting a microbial source as seen in ciguatera where the dinoflagellate Gambierdiscus toxicus produces ciguatoxins and maitotoxin. Ostreopsis siamensis, closely taxonomically related to G. toxicus and producing potent toxins named ostreocins, drew our attention. We report the major ostreocin as a palytoxin analog. O. siamensis (SOA1 strain) was collected at Aka Island, Okinawa, Japan, cultured for 30 days, harvested, and extracted with MeOH/H₂O (1:1) and MeOH/H₂O/HOAc (50:50:0.2). Ostreocins were purified via solvent partition and column chromatography, resolving into five constituents, with the major one (ostreocin D) accounting for 70% of total toxicity (3.8 mg obtained from 936 L of culture). Ostreocin D (C₁₂₇H₂₂₀N₃O₅₃) is a colorless amorphous solid, positive to ninhydrin, with LD₅₀ in mice of 0.75 μg/kg ip, [α]²³ᴰ +22°, UV maxima at 234 and 263 nm (sharing chromophores with palytoxin), and TLC Rf values comparable to palytoxin. Structural analysis via DQF-COSY, TOCSY, HSQC, HMBC, FABMS, and ESIMS identified it as 3,26-bisdesmethyldeoxypalytoxin (missing two methyls and one hydroxyl of palytoxin). Compared to palytoxin, ostreocin D showed reduced cytotoxicity (IC₅₀ against P388 cells: 2.5 μM vs 0.2 μM) and hemolytic potency (complete hemolysis of 0.5% mouse blood cell suspension: 39.5 nM vs 1.5 nM). Palytoxin was not identified in the study, and whether O. siamensis is a symbiont of Palythoa spp. remains unclear. Nonetheless, this is the first study to shed light on the etiology of palytoxin, paving the way for future biosynthetic studies. The search for a palytoxin-producing strain and structural refinement of ostreocin D and its minor congeners are underway.