This study comprises the antioxidant activities, phenolic constituents and essential oil composition of Marrubium heterodon (Benth.) Boiss. & Balansa, an endemic species distributed in Turkey. Essential oil was obtained by hydrodistillation from the aerial parts, and analysed by gas chromatography (GC) and GC/mass spectrometry (MS). In total, 38 components representing 86.8% of the oil were determined. The major components of the essential oil were alpha-pinene (17.3%), myrtenal (11.9%) and alpha-terpineol (5%). The phenolic composition of the aqueous, methanolic and ethyl acetate extracts were evaluated by liquid chromatography (LC)-MS/MS. The antioxidant activity of each extract was determined by using the in vitro 2,2-diphenyl-2-picrylhydrazyl (DPPH) radical scavenging, 2,2'-Azino-bis-3-ethylbenzothiazoline-6-sulfonic acid (ABTS) radical cation decolorization and Copper (II) reducing antioxidant capacity (CUPRAC) assays. To the best of our knowledge, this is the first report on the antioxidant activities, phenolic constituents and essential oil composition of M. heterodon.