Thyme-like plants including Thymbra spicata L. are widely used as food and folk medicinal remedies in the Mediterranean area. This study aimed to explore the in vitro antitumor potential of polyphenol-enriched extracts from aerial parts of T. spicata. The ethanolic extract significantly inhibited proliferation of different human tumor cell lines, without significant effects on non-neoplastic cells. A deeper investigation of the molecular mechanism sustaining the in vitro antitumor activity of the extract was carried on the human breast cancer cells MCF-7 in comparison with the normal breast cells MCF-10A. The effects on MCF-7 cells were associated with the following: (i) production of reactive oxygen species (ROS) and release of nitric oxide; (ii) apoptosis induction; and (iii) reduction in STAT3 and NF-kB phosphorylation. The ethanolic extract from T. spicata leaves might represent a novel therapeutic tool in combination with conventional chemotherapy to reduce the adverse side effects and drug resistance.