The Old Prussians or Baltic Prussians are an ethnic group, autochthonous Baltic tribes that inhabit Prussia, the lands of the southeastern Baltic Sea in the area around the Vistula and Curonian Lagoons.

They speak a language now known as Old Prussian and follow pagan Prussian mythology.During the 13th century, the Old Prussians are conquered by the Teutonic Knights, and gradually assimilate over the following centuries.

The former German state of Prussia takes its name from the Baltic Prussians, although it is led by Germans who have assimilated the Old Prussians; the old Prussian language is extinct by the 17th or early 18th century.

The land of the Old Prussians consists approximately of central and southern East Prussia — the present-day Warmian-Masurian Voivodeship of Poland, the Kaliningrad Oblast of Russia, and the southern Klaipėda Region of Lithuania.