The ctenophore Mnemiopsis leidyi A. Agassiz, 1865 originates from estuaries and coastal waters along Atlantic coastlines from the Chesapeake Bay, USA, to Argentina. Through ballast waters, the species has spread the past 30 years over Eurasia. On the French Mediterranean coastlines, Mnemiopsis leidyi was first formally identified in the Berre Lagoon in 2005 and has been episodically present in other Mediterranean lagoons as Bages-Sigean, Vaccarès, and Estomac. Since 2010–2011 no update of the distribution of Mnemiopsis leidyi has been made. Since 2019, many records of Mnemiopsis leidyi have been reported in other French Mediterranean lagoons along the coast of the Gulf of Lion. In this context, a field survey on the French Mediterranean coast was conducted in July 2020 to measure the expansion of the Mnemiopsis population in coastal lagoons which are potential refuge habitats for this species. Mnemiopsis leidyi was present in 8 different areas: Or, Pérols, Vic, Ingril, Rhône-Sète Channel, Bages-Sigean, and Salses-Leucate. We were able to quantify for the first time the abundance and size structure of Mnemiopsis in the lagoons of Or, Pérols, Vic, Ingril, Rhône-Sète Channel, and Salses-Leucate. A decrease in the abundance of Mnemiopsis leidyi was observed from 1 ind m−3 (Ingril Lagoon) to 28 ind m−3 (Grec Lagoon) and 42 ind m−3 (Rhône-Sète Channel) between the North and the South along the Rhône-Sète Channel in parallel to the increase of salinity (from 20 to 41). The abundances measured in Bages-Sigean and Salses-Leucate lagoons were similar: 6 ind m−3 and 7 ind m−3 respectively.