Old Prussian is an extinct West Baltic language belonging to the Baltic branch of the Indo-European languages, which was once spoken by the Old Prussians, the Baltic peoples of the Prussian region. The language is called Old Prussian to avoid confusion with the German dialects of Low Prussian and High Prussian and with the adjective Prussian as it relates to the later German state. Old Prussian began to be written down in the Latin alphabet in about the 13th century, and a small amount of literature in the language survives. In modern times, there has been a revival movement of Old Prussian, and there are families which use Old Prussian as their first language.

Old Prussian is an Indo-European language belonging to the Baltic branch. It is considered to be a Western Baltic language.

Old Prussian was closely related to the other extinct West Baltic languages, namely Sudovian, West Galindian and possibly Skalvian and Old Curonian. Other linguists consider Western Galindian and Skalvian to be Prussian dialects.

It is related to the East Baltic languages such as Lithuanian and Latvian, and more distantly related to Slavic. Compare the words for 'land': Old Prussian semmē [zemē], Latvian: zeme, Lithuanian: žemė, Russian: земля́, (zemljá) and Polish: ziemia.^{[citation needed]}

Old Prussian had loanwords from Slavic languages (e.g., Old Prussian curtis [kurtis] 'hound', like Lithuanian kùrtas and Latvian kur̃ts, cognate with Slavic (compare Ukrainian: хорт, khort; Polish: chart; Czech: chrt)), as well as a few borrowings from Germanic, including from Gothic (e.g., Old Prussian ylo 'awl' as with Lithuanian ýla, Latvian īlens) and from Scandinavian languages.

The Low German language spoken in Prussia (or West Prussia and East Prussia), called Low Prussian (cf. High Prussian, High German), preserved a number of Baltic Prussian words, such as Kurp, from the Old Prussian kurpe, for shoe in contrast to common Low German: Schoh (Standard German Schuh), as did the High Prussian Oberland subdialect.

Until the 1938 changing of place names in East Prussia, Old Prussian river- and place-names, such as Tawe and Tawellningken, could still be found.

One of the hypotheses regarding the origin of mazurzenie – a phonological merger of dentialveolar and postalveolar sibilants in many Polish dialects – states that it originated as a feature of Polonized Old Prussians in Masuria (see Masurian dialects) and spread from there.