Registered Cossacks (Polish: Kozacy rejestrowi) is the term used for Cossacks (mostly from the Zaporizhian Sich) who were part of the Polish-Lithuanian Commonwealth armies. Registered Cossacks were a part of Commonwealth army from 16th century until the year 1699.

Registered Cossacks formed an elite among Cossacks, serving in the military under commanders (starszy) and main ataman, who were responsible before Great Crown Hetman (Commonwealth highest military commnader). A substantial percent of Cossacks formed skilled light cavalry units (choragwia ), excellent skirmishers trained in mounted archery (and later using firearms), making lighting raids, harrasing heavier, slower formations and disengaging. Those units were often useed as support for heavy elite Commonwealth cavalry, the hussars, and were much cheaper to form that a hussar unit. Cossack units were also known for their tabor formation.

Registered Cossacks had many priviliges, including personal freedom, exclusion from many taxes and duties, and had wages (although Commonwealt military was plagued with fiscal problems, leading to extremly delayed wages, often paid in items like clothes or weapns instead of coin).

Many Cossacks were skilled warriors, and major income of Cossacks came from raids (chadzki) on the southern neigbhours of the Commonwealth (Ottoman Empire and its vassals). However only a small number were actually 'registered Cossacks' - the exact number was from few hundred to few thousands and varied in time, usually being increased during wartime. This has led to many social and political tensions, especially as szlachta (Polish gentry) almost constantly attempted to force Cossacks into submissions as peasants, while Cossacks demanded the significant expantion of the Cossack register. Furthermore, the Cossack-szlachta conflict was aggreviated as Cossacks often supported Commonwealth monarchs (like Wladyslaw IV Waza), who were often at odds with szlachta, wishing to further limit the monarch powers. The tensions between Cossacks and szlachta grew and from the late 16th century resulted in several uprisings (the greatest of which was the Chmielnicki uprising of 1648), with registered Cossacks often forced to chose sides between supporting their own people or the szlachta backed Commonwealth forces.