Olive leaves, an easily available natural low-cost material, constitute a source of extracts with significant antitumor activity that inhibits cell proliferation in several breast-cancer-cell models. In this work, a metabolite-profiling approach has been used to assess the uptake and metabolism of phenolic compounds from an olive-leaf extract in the breast-cancer-cell line SKBR3 to evaluate the compound or compounds responsible for the cytotoxic activity. For this, the extract was firstly characterized quantitatively by high-performance liquid chromatography coupled to electrospray ionization-quadrupole time-of-flight mass spectrometry (HPLC-ESI-QTOF-MS). Then, SKBR3 cells were incubated with 200μg/mL of the olive-leaf extract at different times (15min, 1, 2, 24, and 48h). A metabolite-profiling approach based on HPLC-ESI-QTOF-MS was used to determine the intracellular phenolic compounds, enabling the identification of 16 intact phenolic compounds from the extract and four metabolites derived from these compounds in the cell cytoplasm. The major compounds found within the cells were oleuropein, luteolin-7-O-glucoside and its metabolites luteolin aglycone and methyl-luteolin glucoside, as well as apigenin, and verbascoside. Neither hydroxytyrosol nor any of its metabolites were found within the cells at any incubation time. It is proposed that the major compounds responsible for the cytotoxic activity of the olive-leaf extract in SKBR3 cells are oleuropein and the flavones luteolin and apigenin, since these compounds showed high uptake and their antitumor activity has been previously reported.