Three known triterpenoids, namely ursolic acid (1), and the 27-E- and 27-Z-p-coumaric esters of ursolic acid (compounds 2, 3), were isolated together with a new triterpene 2,3-seco-taraxer-14-en-2,3-lactone [pycanocarpine (4)] from an ethanolic extract of Pleiocarpa pycnantha leaves. The structure of 4 was unambiguously assigned using NMR, HREIMS and X-ray crystallography. The cytotoxic activities of the compounds were evaluated against HeLa, MCF-7, KMST-6 and HT-29 cells using the WST-1 assay. Ursolic acid (1) displayed potent cytotoxic activity against HeLa, HT-29 and MCF-7 cells with IC₅₀ values of 10, 10 and 20 µM respectively. The new compound 4 and its hydrolysed derivative 5 were selectively cytotoxic to the breast cancer cell line, MCF-7 with IC₅₀ values 20 and 10 µM respectively. This is the first report on isolation of a 2,3-seco-taraxerene derivative from the Apocynaceae family and cytotoxic activity of P. pycnantha constituents.