Litosetoenins A-E (1-5), five new ring-rearranged serrulatane-type diterpenoids with a common tricyclo[3.0.4]decane core, along with a known diterpenoid glycoside (6), a related known diterpenoid (7), and four known sesquiterpenoids (8-11), were isolated from a Balinese soft coral Litophyton setoensis. Spirolitosetoenin A (5a) and isospirolitosetoenin A (5b), featuring an unprecedented spiro[4,5]decane core, were obtained after treatment of compound 5 with HCl in methanol. The structures of new compounds were elucidated by extensive spectroscopic analysis, quantum mechanical nuclear magnetic resonance approach, and chemical methods. A plausible biosynthetic pathway involving an unusual divergent biogenesis was proposed.