The estuarine nudibranch Tenellia adspersa (Nordmann, 1845) was recorded for the first time in Portugal, while sampling for fouling fauna of artificial structures along the salinity gradient of the Guadiana estuary (SW Iberian Peninsula). Two specimens were found in association with the invasive hydrozoan Cordylophora caspia (Pallas, 1771) and kept in the laboratory for taxonomy purposes. After two days, batches of eggs were seen in C. caspia branches, while the nudibranchs were also actively feeding on the hydrozoan polyps. The fast generation times of T. adspersa, along with its food preference for C. caspia, might suggest a positive role of the nudibranch on controlling this invasive hydrozoan in the Guadiana estuary. Introduction routes and facilitation interactions are discussed.