A new species endemic to Mitiaro, Cook Islands, Tetramolopium mitiaroense, is described and illustrated. The species is most closely related to Hawaiian species of Tetramolopium section Tetramolopium. Tetramolopium mitiaroense shares an upright dwarf shrub habit with T. remyi and T. capillare but differs in possessing semi-succulent, linear-oblanceolate leaves versus sclerophyllous, involute, linear leaves for the latter two taxa. Tetramolopium sylvae also has succulent leaves but differs from T. mitiaroense in having a prostrate, rosette habit and spatulate leaves. A key is provided for all eastern Pacific species of Tetramolopium. Published morphological, molecular, and geological data are reviewed to assess relationships amongst Tetramolopium taxa in the eastern Pacific and elucidate the biogeographic pattern of dispersal. The data support the hypothesis that Tetramolopium was first dispersed to the Hawaiian Archipelago from New Guinea with a secondary dispersal event from the Hawaiian Islands to the Cook Islands. Available evidence indicates that the Pacific Golden-Plover is the probable agent of dispersal from the Hawaiian Islands to the Cook Islands.