The agglutinated foraminifera Trochammina hadai Uchio (1962), a dominant species in Japan, first appeared as an invasive species in San Francisco Bay, US, in 1983. Trochammina hadai´s first appearance in the Brazilian coastal waters of Flamengo Inlet, Ubatuba, São Paulo State, is recorded nearly three decades later, in two of thirty-four sediment samples collected in the western portion known as “Saco do Ribeira” in 2010. By 2014 and 2015, the species was found alive in 78% of the samples with relative abundance reaching 18%.We suggest this recent colonizationwas most likely the result of an introduction by anchor mud on commercial fishing boats and recreational vessels or from fishing bait, and the species’proliferation due to its suitability to live in anthropogenically-induced stressed environmental conditions of the inlet that has affected the ecological dominance of this benthic community.