The princeps senatus (plural principes senatus) was the leader of the Roman senate. Although officially out of the cursus honorum and owning no imperium, this office brought enormous prestige to the magistrate holding it.

The princeps senatus was not a lifetime job. He was chosen by every new pair of censors (that is, every five years). Censors could, however, confirm a princeps senatus for a period of another five years. He was selected from patrician senators with consular rank, usually former censors. The successful candidate had to be a patrician with an impeccable political record, respected by his fellow senators.

The office tasks include:

• Declaring opening and closure of the senate sessions
• Deciding the agenda
• Deciding where the session should take place
• Imposing order and other rules of the session
• Meeting, in the name of the senate, with embassies of foreign countries
• Writing, in the name of the senate, letters and dispatches

After the fall of the Roman Republic, the princeps senatus was the Emperor (see also: princeps). However, during the Crisis of the Third Century, some others held the office; the future emperor Valerian held the office in 238, during the reigns of Maximinus Thrax and Gordian I.

Incomplete list of principes senatus

• …
• 214 BC – Marcus Fabius Buteo
• 209 BC – Quintus Fabius Maximus Verrucosus
• 199 BC – Publius Cornelius Scipio Africanus Major
• 184 BC – Lucius Valerius Flaccus
• 179 BC – Mamercus Aemilius Lepidus
• 147 BC – Publius Cornelius Scipio Nasica
• 136 BC – Appius Claudius Pulcher
• 131 BC – Lucius Cornelius Lentulus Lupus
• 125 BC – Publius Cornelius Lentulus
• 115 BC – Marcus Aemilius Scaurus
• 86 BC - Lucius Valerius Flaccus
• …