It is similar in plumage to the scarlet robin of Australia, and until recently the two were considered conspecific until split in 1999 by Schodde and Mason.

[5] The Pacific robin was originally described by German naturalist Johann Friedrich Gmelin in 1789 from a collection in Norfolk Island.

The nominate subspecies of Pacific robin, found in Samoa, was described by American naturalist Titian Peale in 1848.

[8][9] Sibley and Alquist's DNA-DNA hybridisation studies placed the robins in a Corvida parvorder comprising many tropical and Australian passerines, including pardalotes, fairy-wrens, and honeyeaters, as well as crows.

[10] However, subsequent molecular research (and current consensus) places the robins as a very early offshoot of the Passerida ("advanced" songbirds).

[11] The Pacific robin is a small passerine, 11.5–13.5 cm long and weighing 9–11 g.[4] Over much of its range, it is the smallest species of bird.

The males of P. m. polymorpha of Makira in the Solomon Islands have two different plumage morphs, including one with no white on the forehead, but with an all rufous-brown head.

A fossil found on the islands of Ha'apai in Tonga shows that the species once occurred in the group, but is now extinct there.

The Pacific robin is not considered globally threatened, and is classified as least concern on the IUCN Red List .