This research evaluated the in vitro apoptotic inducing properties of a methanol leaf extract of carob tree (Ceratonia siliqua L.) on a human cervical adenocarcinoma cell line (HeLa). The cell viability effect on a prostate (DU-145), breast (MDA-MB-231) and colon cell line (HCT-166) was also assessed. The effect of the extract on reactive oxygen species (ROS) production by HeLa cells was studied, and a phytochemical evaluation was made by high performance liquid chromatography with diode array detection (HPLC). Total mean yield of identified phenols was 261.1 mg/g DW, and (+)-catechin was the major compound (76.8 mg/g DW). The extract decreased cell viability in a dose-and time-dependent manner, and a more pronounced effect on HeLa line was observed. In vitro cytotoxic activity was associated with apoptosis, but not to the increase of ROS production. Among the tested compounds identified in the extract, the highest activity was detected with gallic acid (GA), (+)-catechin and quercetin, with reductions in HeLa cells viability down to 8.0, 11.9 and 27.1%, respectively. This is the first report on the apoptotic activity of a leaf extract of carob tree on a human cancer cell line, suggesting that it may be a potential source of chemopreventive compounds.