Isidore of Kiev, also known as Isidore of Thessalonica (Greek: ?s?d???? t?? ???ß??; Russian: ??????; Ukrainian: ??????; b.
Peloponnesus, 1385 – d.Rome, 27 April 1463) was a Byzantine Greek Metropolitan of Kiev, cardinal, humanist, and theologian.
He was one of the chief Eastern defenders of reunion at the time of the Council of Florence.Isidore became the last hierarch of the Old Russian Church before it split into the Russian Eastern Orthodox Church of the Grand Duchy of Moscow and Ruthenian Eastern Orthodox Church of the Grand Duchy of Lithuania.
Following his arrest, the Grand Duke Vasily II of Moscow initiated a schism by installing Metropolitans in Moscow without the approval of the Ecumenical Patriarch of Constantinople.