Gaius Suetonius Tranquillus, commonly known as Suetonius, was an equestrian and a historian during the Roman Empire. His most important surviving work is a set of biographies on the lives of twelve successive Roman rulers, from Julius Caesar until Domitian, entitled De Vita Caesarum. Other works by Suetonius concern the daily life of Rome, politics, oratory, and the lives of famous writers, including poets, historians, and grammarians. A few of these books have partially survived, but many are entirely lost.

Suetonius was born the son of Suetonius Laetus, who probably came from Hippo Regius (Annaba, Algeria). Laetus was an equestrian who served and took part in the first Battle of Bedriacum for the Emperor Otho and against the future Emperor Vitellius in 69.

Suetonius was a close friend to Senator and letter-writer Pliny the Younger. Pliny describes him as "quiet and studious, a man dedicated to writing." Pliny helped him buy a small property in Italy and interceded with the Emperor Trajan to grant Suetonius immunities usually granted to a father of three, the ius trium liberorum, because his marriage was childless. Through Pliny, Suetonius came into favour with Trajan and Hadrian.

Suetonius served on Pliny's staff when Pliny was Proconsul of Bithynia Pontus (northern Asia Minor) between 110 and 112. Under Trajan he served as secretary of studies (precise functions are uncertain) and director of Imperial archives. Under Hadrian, he became the Emperor's secretary. In 122, Hadrian dismissed him for disrespectful behaviour towards Empress Vibia Sabina.

Suetonius may have later regained imperial favor under Hadrian and returned to his position. This hypothesis is based on the suggestion that Offices of State was one of his last works, and that the subject was chosen to reflect Hadrian's administrative reforms. However, there is no certain evidence for a public career after 122.