In the framework of a wider systematic survey aimed at studying non-indigenous species in Italian harbours and marinas, the ascophoran bryozoan Celleporaria brunnea was detected for the first time in the western Mediterranean Sea. The species is presumably native to the Pacific coasts of North America and is distributed from British Columbia to the Galapagos Islands (Ecuador). In the Mediterranean Sea, C. brunnea was first recorded in 2004 in the inner part of Izmir Bay near Alsancak harbour (Turkey) and later reported along the Lebanese coasts. The species was recently detected in Cascais marina in Portugal and in the Arcachon Basin (France), documenting the first records in the eastern Atlantic. The finding in the Italian harbours of La Spezia (Liguria), Olbia (Sardinia) and Lampedusa (off Sicily) marks its western and northernmost occurrence within the Mediterranean basin. Pathways of introduction into the western Mediterranean Sea are discussed, concluding that hull fouling is the most likely vector. The species may be expected to appear soon in other harbours of the Mediterranean basin.