Sideritis perezlarae is a plant widely used in folk medicine in the South of Andalusia (Cádiz, Spain). In this work, a phytochemical analysis has led to the isolation and identification of the flavonoid 7-O- β -glucosyl-luteolin from a methanol extract. The total phenol content estimated by Folin-Ciocalteau assay and expressed as gallic acid equivalent per gram of dried fraction, was 102.54 ± 2.10 mg phenols per gram dry residue. The flavonoid content, investigated by AlCl3 reagent, was 23.49 ± 0.90 mg flavonoids gram dry residue. The methanol extract has been evaluated for antioxidant (DPPH and TEAC assays) and cytotoxic (SRB assay) properties. In the DPPH radical scavenging assay, the IC50 was 360 µg mL(-1). In the total antioxidant activity, calculated by the Trolox equivalent antioxidant activity (TEAC, mg g(-1) of dried fraction), the extract showed a high antioxidant capacity (TEAC value of 0.59 ± 0.02 mg g(-1)). The cytotoxic activity of the extract against a human adenocarcinoma cell line HT-29 presented an IC50 = 69.47 ± 4.64 µg mL(-1).