The slender-billed prion was formally described in 1912 by the Australian born ornithologist Gregory Mathews under the binomial name Heteroprion belcheri.

[4] The name prion comes from the Greek word priōn, meaning "saw", a reference of the serrated edges of the birds' saw-like bill.

Prions produce a stomach oil made up of wax esters and triglycerides that is stored in the proventriculus, which they use against predators as well as an energy rich food source for chicks and for the adults during their long flights.

[8] Finally, they also have a salt gland, situated above the nasal passage, which helps desalinate their bodies by excreting a high saline solution from their nose, relieving excessive salt for their metabolism as they imbibe a high volume of salty ocean water.

Their tail is wedge-shaped and grey with a black tip, their bill is blue-grey, and their feet are pale blue.