Background: Haematoxylum brasiletto is a tree that grows in Central America, commonly known as “Palo de Brasil,” which is used in the traditional medicine for the treatment of cancer and gastric ulcers. Objective: The aim of this study was to isolate the compounds responsible for antiproliferative activity of H. brasiletto. Materials and Methods: A bioassay-guided fractionation of ethanol extract of H. brasiletto was performed using 3-(4, 5-dimethylthiazol-2-yl)-2, 5-diphenyltetrazolium bromide cell proliferation assay to measure the antiproliferative activity on six human cancer cell lines (A549, LS180, HeLa, SiHa, MDA-MB-231, and NCI-H1299) and one human noncancer cell line (ARPE-19). The ethanol extract was partitioned with hexane, dichloromethane, and ethyl acetate. The active dichloromethane fraction was fractioned by silica-column chromatography, and active subfractions were separated using preparative-thin layer chromatography. The chemical structure of an isolated compound was elucidated with different chemical and spectroscopic methods. Results: The flavonoid brazilin (1) was isolated from the heartwood of H. brasiletto. The measurement of antiproliferative activity showed that brazilin can inhibit the growth of SiHa, MDA-MB-231, A549, and NCI-H1299 cell lines by 50% at doses of 44.3, 48.7, 45.4, and 48.7 μM, respectively. Furthermore, the flavonoid showed a high antiproliferative activity on LS 180 and HeLa with IC50 values of 62.2 and 71.9 μM, respectively. Brazilin also exhibited a high antiproliferative activity on the human noncancer cell line ARPE-19 with an IC50 value of 37.9 μM. Conclusions: Brazilin: (6aS,11bR)-7,11b-Dihidro-6H-indeno[2,1-c] cromeno-3,6a, 9,10-tetrol was isolated; this compound demonstrated antiproliferative activity against several human cancer cell lines. This work demonstrated that brazilin, a flavonoid isolated and characterized of H. brasiletto, has antiproliferative activity against cancer cell lines.