| Conflict: | Byzantine civil war of 1352–1357 |
| Partof: | the Byzantine civil wars, the Byzantine–Serbian wars and the Byzantine–Turkish wars |
| Date: | 1352–1357 |
| Place: | Thrace and Constantinople |
| Territory: | Byzantine ruled by John V Palaiologos |
| Result: | John V Palaiologos becomes sole ruler, deposes the Kantakouzenoi |
| Combatant1: | John V Palaiologos Allies: Serbian Empire Second Bulgarian Empire Republic of Venice Republic of Genoa |
| Combatant2: | John VI Kantakouzenos Matthew Kantakouzenos Allies: Ottoman Empire |
The Byzantine civil war of 1352–1357 was an armed conflict resulting from and following the Byzantine civil war of 1341–1347. The war pitted Byzantine emperor John V Palaiologos against John VI Kantakouzenos and his eldest son Matthew Kantakouzenos. John V emerged victorious as the sole emperor of the Byzantine Empire, but the destruction brought about by the civil war left the Byzantine state in ruins.
In the aftermath of the 1341–1347 conflict, John VI Kantakouzenos established himself as senior emperor and tutor over the young John V Palaiologos. However, this state of affairs did not last long. Supporters of the Palaiologoi still distrusted him, while his own partisans would have preferred to depose the Palaiologoi outright and install the Kantakouzenoi as the reigning dynasty. Kantakouzenos' eldest son, Matthew, also resented being passed over in favour of John V. To placate him, a semi-autonomous appanage covering much of western Thrace was created. It doubled as a march against the new Serbian Empire of Stephen Dushan.
Steadily deteriorating relations between Matthew, who now ruled eastern Thrace, and John V, who lived in western Thrace, sowed the seeds for the resumption of the civil war.
Open warfare broke out in 1352, when John V, supported by Venetian and Serbian troops, launched an attack on Matthew Kantakouzenos. John Kantakouzenos came to his son's aid with 10,000 Ottoman troops who retook the cities of Thrace, plundering them in the process. In October 1352, at Demotika, the Ottoman force met and defeated 4,000 Serbians provided to John V by Dushan.[1] This was the Ottomans' first victory in Europe. Two years later, their capture of Gallipoli marked the beginning of the Ottoman conquest of the Balkans, culminating a century later in the Fall of Constantinople. Meanwhile, John V fled to the island of Tenedos, from where he made an unsuccessful attempt to seize Constantinople in March 1353.
John VI Kantakouzenos responded by having Matthew crowned as co-emperor, but John V Palaiologos, enlisting Genoese support and relying on the declining popularity of Kantakouzenos, succeeded in entering the capital in November 1354. John VI Kantakouzenos abdicated and retired to a monastery. Matthew held out in Thrace and made war upon the Serbians in 1356. Later, Matthew gathered an army of 5,000 Turks and marched on Serres, the Serbian-held capital of John Ugleisha. Stephen Urosh V, whose mother also ruled at Serres, decided to raise an army to defend his mother. In 1357, when Matthew and his Turks attacked, the Serbian army under Vojin, Count of Drama (a major fortress in that vicinity) came to the rescue and the Turks were defeated. Matthew was captured and held hostage until his ransom was paid by John V Palaiologos, who was now the sole master of a rump state. Matthew was allowed to go to the Morea and reign there with his brother Manuel Kantakouzenos.