Banksia plagiocarpa, commonly known as Dallachy's blue banksia, blue banksia or Hichinbrook banksia, is a species of shrub or tree in the plant genus Banksia. It occurs only on Hinchinbrook Island, Queensland, and the immediately adjacent mainland. First collected in 1867, Banksia plagiocarpa was not described until 1981, when Alex George named it in his monograph of the genus Banksia. Genetic studies show it to be related to Banksia aquilonia, Banksia oblongifolia and Banksia robur.