As part of research on the Asteraceae family, the chemical constituents of aerial parts of Achillea ligustica All. collected in Jijel, eastern Algeria in June 2004 were studied. Air-dried flowering aerial parts were extracted with ethanol using a Soxhlet apparatus, and the extract was fractionated by column chromatography (silica gel, Sephadex LH-20) with various eluents (hexane-EtOAc, CH₂Cl₂-Me₂CO, etc.). A new chlorinated guaianolide, named algerianolide, was isolated along with several known compounds: (E)-ethyl-3,7-dimethyl-3,6-octadienoate, borneol, 2-oxoisodauc-5-en-12-al, 1-tricontanol, β-sitosterol, filifolide A, and santin. The structure of algerianolide was identified as 2α-chloro-9α-acetoxy-1β,10α-dihydroxy-3β,4β-epoxy-5α,7α-H-guaia-11(13)-en-12,6α-olide through spectral analysis (HR-EI-MS, 2D NMR including COSY, ROESY, HSQC, HMBC) and confirmed by X-ray diffraction. Chemotaxonomic significance analysis revealed that this is the first report of a sesquiterpene lactone with a chlorine atom at C-2 in the Achillea genus, differing from previous findings where chlorine was at C-1, C-3, or C-4.