Quasi-Sovereign was an unofficial title given to various Imperial Viceroys, especially those with the position of Prime Minister, who assumed de facto leadership of the Star Imperium when there was no ruling Imperial Sovereign.  Prominent Quasi-Sovereigns in the Imperium included Dana Atrosos, Hida Atrosos, and various members of the Lental family.

After the establishment of the Council of Ministers in Year 3465, the informal title of Quasi-Sovereign was always granted to the Imperial Viceroy who served as Prime Minister when the office of Imperial Sovereign was vacant.  Many members of the Monarchist Party often satisfied themselves with the leadership of a Quasi-Sovereign when squabbles between Blue Monarchists and Red Monarchists and the growing party of Freethinkers in the Imperial Parliament made it difficult for a Parliament to elect a true Imperial Sovereign.