Demetrios I Kantakouzenos (Greek: Δημήτριος Καντακουζηνός) was a governor of the Morea and the grandson of Emperor John VI Kantakouzenos. Demetrios was the son of Matthew Kantakouzenos, governor of the Morea, and Irene Palaiologina. Demetrios was given the title of sebastokrator by Emperor John V Palaiologos in December 1357 and went to the Peloponnese with his father and grandfather in 1361.
One of the two sons of Matthew Kantakouzenos, he disputed the succession to the Despotate of the Morea with Theodore I Palaiologos, the son of John V between 1380 and 1384. Our only information for this event is a cryptic reference in the Emperor Manuel II Palaiologos' Funeral Oration for his brother Theodore, who remarks on the insubordination of the "son" of Matthew Kantakouzenos, who had usurped the government on the death of Manuel Kantakouzenos in 1380. The traditional view is that this son was John, not Demetrios; however D.A. Zakythenos, a historian of the Despotate of the Peloponnese, was inclined to believe that the son was Demetrios. According to the Byzantinist Donald Nicol, "This problem can hardly be satisfactory solved on the basis of the documentary evidence available".
His exact number of children or further descendants is unknown. The list of possible children includes:
- George Palaiologos Kantakouzenos, "Sachatai" (d. circa 1456–59), scholar and military commander, defended Smederevo during a Hungarian attack in 1456
- Andronikos Palaiologos Kantakouzenos (d. June 3/4, 1453, executed), the last Grand Domestic of the Byzantine Empire
- Thomas Kantakouzenos (d. July 25, 1463 in Adrianople), diplomat for Đurađ Branković, Despot of Serbia
- Irene Kantakouzene (d. May 1457), wife of Serbian Despot Đurađ Branković
- Helena Kantakouzene (d. circa November 1, 1463 in Adrianople), Empress of Trebizond, wife of David Komnenos, Emperor of Trebizond.
- A daughter who married the king of Georgia.