A molecular and morphological taxonomic study of Corallina (Corallinales, Rhodophyta) from Tristan da Cunha and the Falkland Islands revealed Corallina chamberlainiae J.Brodie & R.Mrowicki sp. nov. from both South Atlantic archipelagos, and Corallina cf. caespitosa only in the Falkland Islands. Analysis of mitochondrial COI-5P and plastid psbA resolved C. chamberlainiae as a distinct clade composed of specimens from Tristan and the Falkland Islands (COI-5P), in addition to two matching New Zealand samples (psbA). In psbA analyses, C. cf. caespitosa was close to two species from Japan (as C. pilulifera and C. melobesioides) but was separate from C. caespitosa sensu stricto from Britain and C. ferreyrae (isotype) from Peru. In COI-5P analyses, C. cf. caespitosa was in a clade with C. caespitosa (holotype) and close to, but distinct from, C. ferreyrae and an unidentified South African Corallina species. Application of the Automatic Barcode Gap Discovery (ABGD) and Poisson tree processes (bPTP) for COI-5P lumped C. chamberlainiae into one clade, whereas the Generalized Mixed Yule Coalescent (GMYC) model split it into two well-supported groups, one of which only contained Falkland Island specimens. All three models lumped C. cf. caespitosa with C. caespitosa. C. chamberlainiae closely resembles C. officinalis, the generitype, but is distinguished by smaller size and a more compressed thallus towards the apex. Corallina cf. caespitosa resembles C. caespitosa but is smaller and has fused, palm-like upper fronds. Neither of these taxa is conspecific with C. ferreyrae although C. caespitosa was recently synonymized with C. ferreyrae. C. chamberlainiae and C. cf. caespitosa occupy similar intertidal habitats to their respective, closely related counterparts C. officinalis and C. caespitosa. The results indicate cryptic diversity and suggest that there are many misidentified Corallina species. The paucity of South Atlantic studies of the Corallinales points to the need for a much greater taxonomic effort in this biogeographic region.