A new prenylated xanthone, kaennacowanol D (1), was isolated from the twigs of Garcinia cowa together with 22 known analogs. The structure of the new metabolite was elucidated by extensive spectroscopic analysis, particularly using HRMS and NMR. Kaennacowanol D is the first example of a natural xanthone containing a levulinyl group incorporated at the terminal prenyl unit via ester bond. Cytotoxic evaluation of the isolated compounds on human cancer cells showed that jacareubin (13), 2-prenylisojacareubin (17), nigrolineaxanthone E (23) were significantly active against KB and Hela S3 cell lines with IC50 values ranging from 1.78 to 9.52 µM, while 17 and 23 also suppressed the growth of MCF-7, Hep G2, and HT-29 cells with IC50 values lower than 10 µM.