Princeps ("the first", but usually translated as "First Citizen") was an official title of a Roman Emperor, by some historians seen as the title determining the Emperor in Ancient Rome.

The word "Princeps" derived from Princeps Senatus ("Primus inter pares" 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. While Augustus had political and military supremacy, he needed the assistance of his fellow Romans to manage the Empire.

Ancient Rome knew other kinds of principes too, like princeps iuventutis ("the first amongst the young"), which in the early empire was bestowed on eligible successors to the emperor.

For a comprehensive list of other official Roman titles used for the office of emperor see Roman Emperor. These titles included imperator, Augustus, Caesar, and later dominus ("lord") and basileus (the Greek word for "king"). The word Emperor itself is derived from the Roman title imperator, which was a very high, but not exlusive, military title until Augustus began to use it as his praenomen.

The emperor Diocletian was the first to stop referring to himself as "princeps" altogether, calling himself "dominus", thus dropping even the pretense that being an 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", while with Diocletian began "the Dominate" period.

"Princeps" is the root of the English word prince.