The essential oil (EO) obtained by hydrodistillation of aerial parts of Lebanese Achillea santolinoides subsp wilhelmsii (K. Koch) Greuter was analyzed by GC and GC–MS. The identity of the second major compound was uncertain. Its structure was elucidated using 1D and 2D NMR spectroscopy as well as MS data. It appeared to be a new sesquiterpene with a 1-oxa-bicyclo[5,4,0]-undecane skeleton which was named santolinoidol. The EO revealed interesting antimicrobial potential particularly against Trichophyton species, Staphylococcus aureus and Methicillin-resistant Staphylococcus aureus (MRSA) with MIC values ranging from 16 to 128 μg/ml.