leader (1648–57) of the Zaporozhian Cossacks who organized a rebellion against Polish rule in Ukraine that ultimately led to the transfer of the Ukrainian lands east of the Dnieper River from Polish to Russian control.

Although he had been educated in Poland and had served with Polish military forces against the Turks, Khmelnytsky, who had become chief of the Cossacks at Czyhryn, quarreled with the Polish governor of that region and was forced to flee (December 1647) to the fortress of the Zaporozhian Cossacks, a semimilitary community that had developed from runaway serfs, bandits, and traders who had settled along the Dnieper River. He then organized a rebellion among the Zaporozhian Cossacks and, with the support of the Crimean Tatars, marched against the Poles in April 1648. His victorious advance won him additional support from the dissatisfied peasants, townspeople, and clergy of Ukraine, who joined him in a mass uprising that enabled him to enter Poland proper and seize Lwów (now Lviv) in October 1648.

After winning more victories in 1649, Khmelnytsky made peace with the new Polish king John Casimir, concluding the Compact of Zborów (Aug. 18, 1649); its terms permitted him to establish a virtually independent Cossack principality in Ukraine. The treaty satisfied neither the Polish gentry nor Khmelnytsky's followers, many of whom remained subject to Polish landlords; therefore, he renewed the war in the spring of 1651 but was defeated at the Battle of Beresteczko in June and was compelled to accept a new, less advantageous treaty. He then sought aid from Moscow against Poland and in 1654 directed his Cossacks to take an oath of allegiance to Alexis, the tsar of Russia (see Pereyaslav Agreement).

The Russians subsequently invaded Poland, but Khmelnytsky, not content with his pact with Alexis, entered into secret negotiations with Sweden, which was also at war with Poland. He was about to conclude a treaty with the Swedes, placing the Cossacks under Swedish rule, when he died.

Khmelnytsky sought autonomy for his Cossack followers but succeeded only in devastating their formerly flourishing Dnieper lands and in subjecting them to the rule of Moscow, which gained control of Ukraine east of the Dnieper and gradually curtailed their liberties.