Fifty-six specimens of the red algal genus Martensia from the Main and Northwestern Hawaiian Islands, representing both shallow (36 specimens from 0–20 m depth) and mesophotic habitats (20 specimens from 62–93 m depth), were collected and characterized using morphological and molecular analyses. Phylogenetic analyses of the rbcL gene resolved five distinct clades of Martensia from the Hawaiian Islands, of which none could be attributed to the two previously used taxonomic names for this flora (M. fragilis Harvey and M. flabelliformis Harvey ex J.Agardh). Analyses of the COI barcoding region were consistent with rbcL trends, although fewer specimens amplified and sequenced for this marker. Four rbcL clades (lineages 1–4) were distinct from each other and from other described species of Martensia based on both phylogenetic position and morphological characters, and are here described as new species: Martensia abbottiae A.R.Sherwood & S.-M. Lin sp. nov., Martensia hawaiiensis A.R.Sherwood & S.-M. Lin sp. nov., Martensia tsudae A.R.Sherwood & S.-M. Lin sp. nov., and Martensia lauhiekoeloa A.R.Sherwood & S.-M. Lin sp. nov. Lineage 5 is attributable to M. albida Y.Lee. All five species in the Hawaiian flora occur in mesophotic habitats, while M. hawaiiensis, M. tsudae, and M. albida also inhabit shallow waters of the Main Hawaiian Islands. This study provides additional support for the degree of uncharacterized biodiversity from mesophotic depths in the Hawaiian Archipelago.