Princeps (First Citizen) was the original official title of a Roman Emperor, derived from his position as Princeps Senatus (leader of the Senate). It was first given to the Emperor Augustus in 23 BC, who wisely saw that use of the titles rex (meaning king) or dictator would create resentment amongst senators and other influential men, who had earlier demonstrated their disapproval by supporting the assassination of Julius Caesar. Also, while Augustus had political and military supremacy, he needed the assistance of his fellow Romans to manage the Empire.

Other official Roman titles used for the office of emperor included imperator, Augustus, Caesar, dominus , and basileus. The word Emperor itself is derived from the Roman title Imperator, which was a military title, until Julius Caesar began to use it as his praenomen during his dictatorship.

The Emperor Diocletian was the first to stop referring to himself as "Princeps" altogether, calling himself "Dominus" (lord), thus dropping even the pretense that the Emperor was not truly a royal office. The period when the Emperors that called themselves Princeps ruled, from Augustus to Diocletian, is called "the Principate ", but Diocletian began "the Dominate " period.

"Princeps" is the root of the modern word Prince.