Campnosperma zeylanica Thwaites is a plant endemic to Sri Lanka. Bio-assay guided fractionation of the hexane extract of leaves of C. zeylanica yielded three new cytotoxic compounds identified as alkyl-hydroxycyclohexenones; Campnospermenone A, Campnospermenone B and Campnospermenone C. Structures of the compounds were elucidated using ¹H and ¹³C spectroscopy. Cytotoxic effects of Campnospermenone A, B and C were evaluated in MCF-7 (oestrogen receptor positive), MDA-MB-231 (triple negative) and MCF 10A (normal mammary epithelial) cells by sulforhodamine B (SRB) assay. Caspase 3/7 activation in breast cancer cells exposed to the compounds was also studied. All three compounds showed strong cytotoxicity (IC₅₀ ranging from 1 to 15 μM) in breast cancer cells with less cytotoxicity to normal mammary epithelial cells. Activation of caspase 3/7 was also observed in breast cancer cells exposed to compounds. This study demonstrated that novel compounds isolated from C. zeylanica seem to be mediating potent cytotoxic effects in breast cancer cells through activation of caspase 3/7.