A new diphenacyl-piperidine alkaloid, sonneratine A (2), identified as a piperidine ring bearing two phenacyl substitutes at C-2 and C-6, and a known related derivative, (+/-) 1-(2-piperidyl)-4-(p-methoxyphenyl)-butanone-2 (3), were isolated from the leaves and stems of the Hainan mangrove Sonneratia hainanensis. The structure of the new compound was determined by extensive analysis of its spectroscopic data and by comparison of its NMR data with those reported in the literature.