A second species in the siphonous green algal genusAvrainvilleawas recently discovered off the island of O‘ahu in the Main Hawaiian Islands. Specimens were collected from Honolulu Harbor, including its entrance channel, and near Ke‘ehi Harbor. These locations are both in Mālama Bay on O‘ahu’s south shore in or adjacent to urbanized estuaries, respectively.In situobservations, morphological and molecular assessments were conducted to examine the alga’s habit and distribution, as well as to assess its putative species identification.The alga occurred in sand as single individuals or in clusters of several individuals at both sites, and near or within seagrass beds (Halophiladecipiens) and algal meadows composed of the green algaHalimedakanaloanaand an unidentifiedUdoteaspecies at the Ke‘ehi Harbor site. All analyses supported both populations as representative of the same taxa, reported until further investigation in the broad Pacific as Avrainvilleacf.erecta based on morphological and molecular analyses. This record of a secondAvrainvilleaspecies in Hawai'i is of particular concern considering that an alga recognized asA.amadelpha, first observed in 1981 from two locales on O‘ahu’s south shore, has become invasive in Hawai‘i’s intertidal to mesophotic environments.