Wessex Archaeology was commissioned by West Dorset District Council (WDDC), under guidance from Jeremy Benn Associates Ltd (JBA Ltd), to undertake an archaeological watching brief and geoarchaeological monitoring of site investigation works at the Cobb, Lyme Regis, West Dorset. Site investigation works involved seven hand dug test pits, five machine dug test pits and four window sampled boreholes located in and around the harbour. No deposits of palaeoenvironmental or geoarchaeological significance were found to be present; with storm beach deposits (aka marine beach shingles) found to overlie sedimentary mudstones. However, earlier phases of the Cobb itself were observed and recorded. The monitoring of the machine dug trial pits recorded a section of 16th century Cowstone foundations on the north side of the Southern Arm and a single displaced Portland Limestone block with characteristic dovetail joint dating it to a repair of 1785. No other archaeological artefacts or features were found. However, this study has shown that future work close to the existing Cobb walls will need to be carried out with care.