Sarmatism (or Sarmatianism; Polish: Sarmatyzm; Lithuanian: Sarmatizmas) was an ethno-cultural identity within the Polish–Lithuanian Commonwealth. It was the dominant Baroque culture and ideology of the nobility (szlachta) that existed in the time from the Renaissance to the early 18th century. Together with the concept of "Golden Liberty", it formed a central aspect of the Commonwealth social elites’ culture and society. At its core was the unifying belief that the people of the Polish–Lithuanian Commonwealth descended from the ancient Iranian Sarmatians, the legendary invaders of contemporary Polish and Roman lands in antiquity.

The term and culture were reflected primarily in 17th-century Polish literature, as in Jan Chryzostom Pasek's memoirs and the poems of Wacław Potocki. The Polish gentry wore a long coat, called kontusz, knee-high boots, and carried a szabla (sabre), usually a karabela. Moustaches were also popular, as well as decorative feathers in men's headgear. Poland's "Sarmatians" strove to achieve martial skill on horseback, believed in equality among themselves, and in invincibility in the face of the enemy. Sarmatism lauded past victories of the Polish military, and required Polish noblemen to cultivate the tradition.

Sarmatia (Polish: Sarmacja) was a semi-legendary, poetic name for Poland that was fashionable into the 18th century, and which designated qualities associated with the literate citizenry of the vast Polish–Lithuanian Commonwealth. Sarmatism greatly affected the culture, lifestyle and ideology of the Polish nobility. It was unique for its cultural mix of Oriental, Western and native traditions. Criticized during the Polish Enlightenment, Sarmatism was rehabilitated by the generations that embraced Polish Romanticism. Having survived the literary realism of Poland's "Positivist" period, Sarmatism made a comeback with The Trilogy of Henryk Sienkiewicz, Poland's first Nobel laureate in literature.

The term Sarmatism was first used by Jan Długosz in his 15th century work on the history of Poland. Długosz was also responsible for linking the Sarmatians to the prehistory of Poland and this idea was continued by other chroniclers and historians such as Stanisław Orzechowski, Marcin Bielski, Marcin Kromer, and Maciej Miechowita. Miechowita's Tractatus de Duabus Sarmatiis became influential abroad, where for some time it was one of the most widely used reference works on the Polish–Lithuanian Commonwealth. The idea appeared due to the humanists' romantic admiration of Antiquity and an attempt to return an outdated onomastics.

According to the Geography by Ptolemy, Sarmatia was considered to be territory of Poland, Lithuania, and Tartary and consisted of Asian and European parts divided by the Don River. As a geographical term, Sarmatia was always indistinct, but very stable. The presumed ancestors of the szlachta, the Sarmatians, were a confederacy of predominantly Iranian tribes living north of the Black Sea. In the 5th century BC Herodotus wrote that these tribes were descendants of the Scythians and Amazons. The Sarmatians were infiltrated by the Goths and others in the 2nd century AD, and may have had some strong and direct links to Poland. The legend of Polish descent from Sarmatians stuck and grew until most of those within the Commonwealth, and many abroad, believed that many Polish nobles were descendants of the Sarmatians (Sauromates). Another tradition came to surmise that the Sarmatians themselves were descended from Japheth, son of Noah.

Some holding to Sarmatism tended to believe that their ancestors had conquered and enserfed the local Slavs and, like the Bulgars in Bulgaria or Franks who conquered Gaul (France), eventually adopted the local language. Such nobility might believe that they belonged (at least figuratively) to a different people than the Slavs whom they ruled. "Roman maps, fashioned during the Renaissance, had the name of Sarmatia written over most of the territory of the Polish–Lithuanian Commonwealth, and thus 'justified' interest in 'Sarmatian roots'."

Centuries later, modern scholarship discovered evidence showing that the Alans, a late Sarmatian people speaking an Iranian idiom, did invade Slavic tribes in Eastern Europe before the sixth century, and that these "Sarmatians evidently formed the area's ruling class, which was gradually Slavicized." Their direct political connection to Poland, however, would remain somewhat uncertain. In his 1970 publication The Sarmatians (in the series "Ancient Peoples and Places") Tadeusz Sulimirski (1898–1983), an Anglo-Polish historian, archaeologist, and researcher on the ancient Sarmatians, discusses the abundant evidence of the ancient Sarmatian presence in Eastern Europe, e.g., the finds of various grave goods such as pottery, weapons, and jewelry. Possible ethnological and social influences on the Polish szlachta would include tamga-inspired heraldry, social organization, military practices, and burial customs.

Sarmatism was used to integrate the Lithuanian and Ruthenian nobles into the Polish-Lithuanian Commonwealth, and also elevated Ukrainian Cossacks as part of this identity despite their non-noble status. Historian Karin Friedrich points out, from the writings of Commonwealth scholars Christoph Hartknoch and Thomas Clagius, that the German-speaking Protestant burghers of Royal Prussia also identified themselves with the ideology of Sarmatism, in particular, with its values of liberty, which they contrasted with Swedish or Imperial (German) identities which they associated with tyranny. Hartknoch claimed that "one European Sarmatia, as one common mother, nurtured the Poles and Lithuanians and Prussians," while Clagius wrote of many nations "diverse in customs, but all by origin from [Sarmatia]."