In October 2015, the dark false mussel, Mytilopsis leucophaeata, was encountered in the Duck Pond of the West River at Edgewood Park in New Haven, New Haven County, Connecticut, USA. The natural endemic range of Mytilopsis leucophaeata is the Gulf of Mexico and the temperate Atlantic coast of North America from Tampico, Mexico, northward to the Chesapeake Bay. In the late 1930s, Mytilopsis leucophaeata became established in the Hudson River basin in New York. The only previous reports of Mytilopsis leucophaeata from New England are from the Charles River near Boston, Massachusetts, in 1981 and from the Housatonic River in Fairfield County, Connecticut, in the 1990s. Mytilopsis leucophaeata has been characterized as the brackish-water equivalent to the zebra mussel, Dreissena polymorpha, and has been associated with severe fouling problems of water-cooling systems in Europe where it has been introduced. It is hypothesized that Mytilopsis leucophaeata was introduced by ballast water of ships from New Haven Harbor and established in the lower West River subsequent to the installation of two-way tidal gates in the course of a tidal marsh and channel restoration project completed in 2012. Other invertebrates collected were the hydrozoan Cordylophora caspia; the polychaetes Hobsonia florida and Polydora cf. ciliata; the amphipods Gammarus fasciatus and Apocorophium simile; the ivory barnacle, Amphibalanus eburneus; and Say's mud crab, Dyspanopeus sayi. The blue crab, Callinectes sapidus, was commonly observed. The freshwater leech, Helobdella modesta, which was once abundant in the Duck Pond, was not collected. Given the invasive potential of Mytilopsis leucophaeata, in conjunction with the paucity of knowledge of many aspects of its natural history, especially in introduced settings, it is important to monitor this population.