Rhamphocottus nagaakii is a species of marine ray-finned fish belonging to the family Rhamphocottidae, the grunt sculpins.

This species was first formally described in 2022 with its type locality given as off Minami-Sanriku cho in Miyagi Prefecture of Japan.

[1] It is thought to have evolved from a common ancestor to the grunt sculpin (Rhamphocottus richardsonii) but the two species were thought to have been separated by a cooling event during either the Pliocene or Miocene.

This species differs from the grunt sculpin genetically and morphologically.

[2] The specific name honours Nagaaki Satoh, a professional diving instructor, who was the first to observe the reproductive behaviour of R. nagaakii and passed these observations to the species' describers.