Oleuropein, a non toxic secoiridoid derived from the olive tree, is a powerful antioxidant and anti angiogenic agent. Here, we show it to be a potent anti cancer compound, directly disrupting actin filaments in cells and in a cell free assay. Oleuropein inhibited the proliferation and migration of advanced grade human tumor cell lines in a dose responsive manner. In a novel tube disruption assay, Oleuropein irreversibly rounded cancer cells, preventing their replication, motility, and invasiveness; these effects were revers ible in normal cells. When administered orally to mice that developed spontaneous tumors, Oleuropein completely regressed tumors in 9 12 days. When tumors were resected prior to complete regression, they lacked cohesiveness and had a crumbly consistency. No viable cells could be recovered from these tumors. These observations elevate Oleuropein from a non toxic antioxidant into a potent anti tumor agent with direct effects against tumor cells. Our data may also explain the cancer protective effects of the olive rich Med iterranean diet.