Tetraplasandra is a genus of seven species endemic to the Hawaiian Archipelago. Recent field studies in the Ko‘olau Mountains on the island of O‘ahu have led to a taxonomic reevaluation of a rare species, Tetraplasandra lydgatei. The species, originally described in the late 1800s, was placed into the widespread, polymorphic species T. oahuensis in a subsequent treatment of the genus. Several morphological characters and varying ecological habitats distinguish the two species. Based on these differences T. lydgatei deserves formal taxonomic recognition. Furthermore, T. lydgatei was an uncommon species even when it was originally described. This may be due to the early human alteration of the dry and mesic Hawaiian forests for housing and agriculture and also that the species was always only an occasional component of the mesic ecosystem. Regardless of the reasons, the rarity of this species has been accelerated. Currently, only six individuals of T. lydgatei are known to exist, and conservation efforts to protect it are needed.