The title of protosebastos (Greek: πρωτοσέβαστος, prōtosébastos, "first sebastos") was a high Byzantine court title created by Emperor Alexios I Komnenos.

Although the title first appears in a document of 1049, where Domenico I Contarini, the Doge of Venice, uses it alongside the title of patrikios to refer to himself, it is commonly accepted that it was created by Emperor Alexios I Komnenos (r. 1081–1118).^[1] It was first conferred to his brother Adrianos,^[2] while another early holder, his brother-in-law Michael Taronites, was soon after raised to the even higher title of panhypersebastos.^[1] It was also conferred on Sergius VI of Naples and his son, John VI, at about the same time.^[3]

Later, during the 12th century, it was given to close relatives of the Byzantine emperor, such as the eldest son of a sebastokratōr.^[4] In the Palaiologan period it was conferred to leading aristocratic families, such as the Tarchaneiotai, the Raoul, etc.^[1]

The Book on Offices by Pseudo-Kodinos, written shortly after the middle of the 14th century, places the prōtosebastos in the thirteenth place in the overall hierarchy after the emperor, between the megas logothetēs and the pinkernēs.^[5] His ceremonial costume comprised a golden-green skiadion hat with silk embroideries, or a domed skaranikon in a reddish apricot colour decorated with gold-wire embroidery, with a painted glass depiction of the emperor standing in front, and enthroned in the rear. A rich silk kabbadion tunic was also worn.^[6]

• Adrianos Komnenos, brother of Alexios I Komnenos
• Alexios Branas, general
• Alexios Komnenos, nephew of Manuel I Komnenos, de facto regent in 1180–82
• Constantine Bodin, ruler of Duklja in 1081–1101
• George Mouzalon, friend and chief minister of Theodore II Laskaris
• Hrelja, magnate of the Serbian Empire
• Teodor I Muzaka, ruler of Principality of Muzaka and Protosebastos
• John Komnenos, nephew of Alexios I Komnenos and governor of Dyrrhachium
• Michael Panaretos, official and historian of the Empire of Trebizond
• Philaretos Brachamios, general
• Theodore Branas, general, son of Alexios Branas and husband of the twice empress Agnes of France

• Kazhdan, Alexander, ed. (1991). The Oxford Dictionary of Byzantium. Oxford and New York: Oxford University Press. ISBN 0-19-504652-8.
• Magdalino, Paul (2002) [1993]. The Empire of Manuel I Komnenos, 1143–1180. Cambridge: Cambridge University Press. ISBN 0-521-52653-1.
• Stiernon, Lucien (1965). "Notes de titulature et de prosopographie byzantines: Sébaste et gambros". Revue des études byzantines. 23: 222–243. doi:10.3406/rebyz.1965.1349.
• Verpeaux, Jean, ed. (1966). Pseudo-Kodinos, Traité des Offices (in French). Centre National de la Recherche Scientifique.
• von Falkenhausen, Vera (2007). "The South Italian Sources". Proceedings of the British Academy. 132.