The Old Polish language (Polish: język staropolski, staropolszczyzna) was a period in the history of the Polish language between the 10th and the 16th centuries. It was followed by the Middle Polish language.

The sources for the study of the Old Polish language are the data of the comparative-historical grammar of Slavic languages, the material of Polish dialects, several Latin manuscripts with Polish glosses, as well as – most importantly – monuments written in Old Polish: the Holy Cross Sermons (Polish: Kazania świętokrzyskie), the Sankt Florian Psalter (Psałterz floriański), Bogurodzica (Bogurodzica), the Bible of Queen Sophia (Biblia królowej Zofii, or Sárospatak Bible, Biblia szaroszpatacka) and some others.

The Old Polish language was spoken mainly on the territory of modern Poland. It was the main vernacular of medieval Polish states under the Piasts and early Jagiellons, although it was not the state language (that being Latin).

The Polish language started to change after the baptism of Poland, which caused an influx of Latin words, such as kościół "church" (Latin castellum, "castle"), anioł "angel" (Latin angelus). Many of them were borrowed via Czech, which, too, influenced Polish in that era (hence e.g. wiesioły "happy, blithe" (cf. wiesiołek) morphed into modern Polish wesoły, with the original vowels and the consonants of Czech veselý). Also, in later centuries, with the onset of cities founded on German law (namely, the so-called Magdeburg law), Middle High German urban and legal words filtered into Old Polish.

Around the 14th or the 15th centuries, the aorist and the imperfect became obsolete. In the 15th century the dual fell into disuse except for a few fixed expressions (adages and sayings). In relation to most other European languages, though, the differences between Old and Modern Polish are comparatively slight, and the Polish language is somewhat conservative relative to other Slavic languages. That said, the relatively slight differences between Old and Modern Polish are unremarkable considering that the chronological stages of other European languages that Old Polish is contemporary with are generally not very different from the Modern stages and many of them already labelled "Early Modern". Old Polish includes texts that were written as late as the Renaissance.

The Book of Henryków (Polish: Księga henrykowska, Latin: Liber fundationis claustri Sancte Marie Virginis in Heinrichau), contains the earliest known sentence written in the Polish language: Day, ut ia pobrusa, a ti poziwai (pronounced originally as: Daj, uć ja pobrusza, a ti pocziwaj, modern Polish: Daj, niech ja pomielę, a ty odpoczywaj or Pozwól, że ja będę mielił, a ty odpocznij, English: 'Let me grind, while you take a rest'), written around 1270.

The medieval recorder of the phrase, the Cistercian monk Peter of the Henryków monastery, noted: Hoc est in polonico 'This is in Polish'.

The difficulty that medieval scribes had to face while attempting to codify the language was the inadequacy of the Latin alphabet to some features of Old Polish phonology, such as vowel length and nasalization, or the palatalization of consonants. Thus, Old Polish did not have a unified spelling. Polish glosses in Latin texts use romanized spelling, which often failed to distinguish between distinct phonemes. Already then, however, certain spellings of proper names become unified.