Neosiphonia echinata, a well-known western Atlantic species, is here reported for the first time from Indonesia and this extends its distribution into Southeast Asia. Neosiphonia echinata was collected from Sulawesi, Indonesia as an epiphyte on Kappaphycus alvarezii and growing on ropes in a culture farm. We also collected this species from Florida, USA, and then compared their morphology and molecular data. Neosiphonia echinata is identified by having erect main filaments arising from prostrate filaments, numerous rhizoids cutting off from the proximal end of pericentral cells by a cross wall, four pericentral cells throughout the thallus, ecorticate axes, very long and abundant trichoblasts more than twice forked, abundant adventitious laterals, procarps with four-celled carpogonial branches, and a spiral arrangement of tetrasporangia. Our phylogenetic analyses of rbcL and cox1 indicate that N. echinata found in Indonesia was very closely related to material from the Western Atlantic Ocean. Neosiphonia echinata may be added as an introduced species to the Indonesian marine flora. The shipping routes western Atlantic-Mediterranean-Indonesia and western Atlantic-Pacific Ocean-Indonesia might be considered as possible pathways of introduction as a consequence of ballast water and hull fouling.