Manuel II Palaiologos or Palaeologus (Greek: Μανουήλ Β΄ Παλαιολόγος) was Byzantine Emperor from 1391 to 1425. Shortly before his death he was tonsured a monk and received the name Matthew. He is commemorated on July 21.
Manuel II Palaiologos was the second son of Emperor John V Palaiologos and his wife Helena Kantakouzene. His maternal grandparents were Emperor John VI Kantakouzenos (1347–1354) and Irene Asanina.
Granted the title of despotēs by his father, the future Manuel II traveled west to seek support for the Byzantine Empire in 1365 and in 1370, serving as governor in Thessalonica from 1369. The failed attempt at usurpation by his older brother Andronikos IV Palaiologos in 1373 led to Manuel's being proclaimed heir and co-emperor of his father. In 1376–1379 and again in 1390 they were supplanted by Andronikos IV and then his son John VII, but Manuel personally defeated his nephew with help from the Republic of Venice in 1390. Although John V had been restored, Manuel was forced to go as an honorary hostage to the court of the Ottoman Sultan Bayezid I at Prousa (Bursa). During his stay, Manuel was forced to participate in the Ottoman campaign that reduced Philadelpheia, the last Byzantine enclave in Anatolia.
Hearing of his father's death in February 1391, Manuel II Palaiologos fled the Ottoman court and secured the capital against any potential claim by his nephew John VII. Although relations with John VII improved, Sultan Bayezid I besieged Constantinople from 1394 to 1402. After some five years of siege, Manuel II entrusted the city to his nephew and embarked (along with a suite of 40 people) on a long trip abroad to seek assistance against the Ottoman Empire from the courts of western Europe, including those of Henry IV of England, Charles VI of France, Sigismund the Holy Roman Emperor, Queen Margaret I of Denmark and king Martin of Aragon.
Manuel II stood on friendly terms with the victor in the Ottoman civil war, Mehmed I (1402–1421), but his attempts to meddle in the next contested succession led to a new assault on Constantinople by Murad II (1421–1451) in 1422. During the last years of his life, Manuel II relinquished most official duties to his son and heir John VIII Palaiologos, and went back to Europe searching for assistance against the Ottomans, this time to the King Sigismund of Hungary, staying for two months in his court of Buda.
Manuel II died on 21 July 1425.