Celtodoryx ciocalyptoides is a Pacific sponge recorded for the first time as invasive in Europe (south Brittany, France) in 1996, then in 2005 (The Netherlands) and 2014 (eastern English Channel), likely in relation with aquaculture activities. It was observed by amateur divers in Arcachon Bay, Hossegor Lake (SW coast of France) and Thau Lagoon (French Mediterranean), but our study formally confirms its presence in the former two locations. Among the associated species, there was another non-indigenous species in both sites, the spionid polychaete Polydora colonia. Described from USA Atlantic coasts and later found in the Mediterranean, this worm seems to live exclusively in association with C. ciocalyptoides in our study sites. We suggest that this unlikely association between a Pacific sponge and a Western Atlantic worm, likely distributed along European coasts by oyster transfers, is probably more widespread than currently known.