Helichrysum microphyllum Cambess. subsp. tyrrhenicum Bacch., Brullo e Giusso (Asteraceae), previously known as Helichrysum italicum ssp. microphyllum (Willd.) Nyman, is one of the many endemic species growing in Sardinia, Corsica and Balearic Islands. In the present work the composition of the essential oil obtained from a population of H. microphyllum ssp. thyrrenicum growing in a littoral location of La Maddalena Archipelago was investigated by GC-FID and CG-MS. The major compounds of the oil were the monoterpene ester neryl acetate (18.2%), the oxygenated sesquiterpene 5-eudesmen-11-ol (rosifoliol, 11.3%), the sequiterpene hydrocarbons δ-cadinene (8.4%) and γ-cadinene (6.7%), showing a peculiar composition in comparison with other Sardinian populations. The oil was tested for cytotoxicity on three human tumor cell lines (MDA-MB 231, HCT116 and A375) by MTT assay showing a strong inhibitory activity on human malignant melanoma cells A375 (IC50 of 16 µg/ml). In addition the oil was assessed for antioxidant activity by DPPH and ABTS assay.