As for many other Azorean Islands, the macroalgal flora of Terceira (belonging to the central group of the archipelago) is poorly known, the published information reflecting occasional collections of sporadic visitors to the island. In order to overcome this and contribute to improve the knowledge of Azorean macroalgal flora at both local and regional scales, a thorough investigation was conducted. Both collections and presence data recordings were undertaken at the littoral and sublittoral levels down to approximately 40 m around the island, covering a total area of approximately 49 km². This paper lists the taxonomic records and provides information on each species’ ecology and occurrence on the Island’s littoral.

A total of 418 specimens (including taxa identified only to genus level) belonging to 147 taxa of macroalgae, comprising 95 Rhodophyta, 33 Chlorophyta and 19 Ochrophyta (Phaeophyceae) are registered. Of these, 113 were identified to species level (73 Rhodophyta, 24 Chlorophyta and 16 Ochrophyta), encompassing 35 new records for the Island (27 Rhodophyta, 6 Chlorophyta and 2 Ochrophyta). Most species are native, including the Macaronesian endemisms Codium elisabethiae O.C.Schmidt, Millerella tinerfensis (Seoane-Camba) S.M.Boo & J.M.Rico and Phyllophora gelidioides P.Crouan & H.Crouan ex Karsakoff. Eight species are introduced and 15 have uncertain origin.