Cancer is among the top global public health burdens leading to millions of deaths each year. The study aims to investigate the antiproliferative effect of Spartium junceum L. flowers on different cancer cell lines. The ethanolic extract of the flowers was prepared in the present study. Phytochemical analysis of the plant extract revealed the presence of several phenolic compounds such as cinnamic acid and its derivatives (chlorogenic, p-coumaric, ferulic acids), protocatechuic acid, epicatechin and luteolin. This extract was tested against human breast (MDA-MB-231) and liver (HepG2) cancer cell lines to find out its antiproliferative activity. It was determined that the extract was effective against both cell lines with IC50 values of 2.37 ± 0.47 and 0.98 ± 0.01 µL/mL for MDA-MB-231 and HepG2, respectively. Particularly, the extract was found to be more effective in the liver cancer cell line than the breast cancer cell line. All these obtained findings led us to believe that this medicinal plant could be a promising antiproliferative agent candidate for the treatment of human liver and breast cancers.