A methanol extract of leaves and twigs from Ardisia iwahigensis demonstrated toxicity toward brine shrimp as well as LNCaP, ZR-75-1, and Lu1 human cancer cells in culture. A novel alkenylphenol, (Z)-1,16-bis(3-hydroxy-5-methoxyphenyl)-10-hexadecene-1,15-dione (ardisenone) (1), was isolated from the extract by bioassay-directed fractionation. This compound demonstrated moderate cytotoxicity against BC1, Lu1, Col2, KB, KB-V1, and LNCaP cell lines.