A first checklist is provided for the lichenised fungi of Mangaia. Based on field work undertaken during November 2024, when 144 collections were made over a 5-day period, 66 species are recognised from Mangaia, including 43 new national records for the Cook Islands. A number of the new records reported here for Mangaia are first reports for islands of the wider Pacific Ocean or south-western Polynesia. The new Mangaia records comprise 37 genera from 22 families. Seven genera include three or more species: Anisomeridium, Cratiria, Dirinaria, Graphis, Physcia, Pyrenula and Ramalina. Eight collections could not be identified to species level. A remarkable specimen is that of Gassicurtia manguensia Marbach which was made from the trunk of Fagraea berteroana A.Grey ex Benth. in Mangaia. Previously this taxon was only known from the type collection from mangrove bark in south-eastern Brazil. An up-to-date Cook Island checklist is provided in which 156 taxa are recognised by combining the new Mangaia records with earlier records mainly from Rarotonga. The most species-rich genera in the Cook Islands are Pertusaria (8 taxa), Porina (11), Pyrenula (13), and Ramalina (8). Keywords: biodiversity | lichenised Ascomycota | new records | Oceania | Pacific Ocean | Polynesia.