The slender-billed prion (Pachyptila belcheri) or thin-billed prion, is a species of petrel, a seabird in the family Procellariidae. It is found in the southern oceans.
Like all prions, they are blue-grey above and white below with a dark "M" on their back to their wingtips. They have a white eyebrow and a dark line extending from below the eye almost to the neck. Their tail is wedge-shaped and grey with a black tip, their bill is blue-grey, and their feet are pale blue.