Lechaion's inner harbour basin was constructed in the 7th-6th c. BCE and served as Corinth's principal port for over a millennium. The harbour decline and abandonment in the 6th c. CE has been attributed to several causes: natural siltation, co-seismic uplift, coastal subsidence, and damage by tsunami impacts. A multi-proxy palaeoenvironmental study was conducted on seven cores from Lechaion's inner harbour to determine changes in the coastal environments and timing and cause of harbour abandonment. Palaeoenvironments were reconstructed using high-resolution micro-XRF core scanning of sedimentary facies, isotopic (δ18O, δ13C), and micropalaeontological analyses (foraminifera, palynomorphs). The harbour lithostratigraphy consists of an uppermost (∼1 m) sequence of laminated mud and marl overlying interbedded pebbly sand and mud containing abundant marine microfossils and Roman pottery refuse. A thin (<12 cm) calcrete layer at the base of the marl defines a basin-wide paraconformity, marking a transition from a marine-estuarine harbour basin to a restricted, evaporitic lake. Basin restriction is recorded by a sharp decline in terrigenous elements (Si, Ti, K, Fe), increased Sr, δ18O, a decline in foraminifera and marine dinoflagellate cysts, and an increase in freshwater algae. The event is constrained by AMS 14C age modelling to the 6th c. CE and interpreted as rapid, co-seismic uplift of the harbour floor, most likely during destructive earthquakes of 524 and 551/552 CE. These seismic events have been linked to a ∼1.1 m uplift of the nearby Perachora Peninsula and sediment liquefaction structures on-site. No evidence was found for 2nd c. BCE or 6th c. CE tsunami events proposed in previous work. This study represents the most comprehensive geoarchaeological study completed to date in Lechaion's inner harbour and confirms its destruction and abandonment in the 6th c. CE as a result of co-seismic uplift and rapid shoaling of the inner basin. •Destruction of Lechaion's inner harbour previously attributed to various causes, including tsunami impacts.•Multi-proxy study conducted on 7 cores from inner harbour basin.•Rapid environmental shift in the 6th c. CE, from a marine-estuarine harbour to shallow, freshwater lake.•Tectonic uplift of harbour basin, tentatively linked to destructive earthquakes of 524 and 551/552 CE.