János Kemény (14 December 1607 in Magyarbükkös – 23 January 1662 in Szásznagyszőllős) was a Hungarian aristocrat, writer and prince of Transylvania.
János Kemény, offspring of a Transylvanian family of Hungarian aristocrats, held various political and military positions in the Principality of Transylvania, during the reign of princes Gábor Bethlen and George I Rákóczi. As he writes, the family descends from Kemeny Simon, who disguised himself as Hunyadi János and was killed by the Turks instead of his master. The story is recorded by Heltai Gaspar's work, Magyar Cronica, a Hungarian translation/rewriting of the Latin of Antonio Bonfini published in 1560s.
Under George II Rákóczi, he became the chief advisor of the prince, and leader of the military campaigns to Moldavia in 1653 and Poland in 1657, the latter being aimed at obtaining the Polish crown for Rákóczi. The Turks strongly opposed to Rákóczi's Polish ambitions and prohibited any military action. In the face of Turkish prohibition, Kemény also objected to the prince's ill-conceived Polish undertaking, and unwillingly took the leadership of the campaign that ended in disaster; Kemény's army, deserted by the Swedish allies, was captured by the Crimean Tatar allies of the Turks.
Kemény was held captive by the Tatars until August 1659, when he was released for a huge ransom. During his captivity in Bakhchisaray he wrote his Autobiography (in Hungarian), one of the most excellent masterpieces of the Transylvanian memoirist literature of the 17th century.
When he returned to Transylvania, he found a country torn by the ambitions of George II Rákóczi, unwilling to resign, a number of would-be princes, and a Turkish Porte vengeful for the unauthorized military campaign. The Sublime Porte now openly disregarded the Transylvanian Estates' right to elect the prince; Kemény saw this, and the successional invasions of Transylvania by the Turks and their Crimean Tatar allies, as an end of Transylvania's autonomy, which he thought could be prevented only with reliance on Habsburg help.