AD 31 (XXXI) was a common year starting on Monday of the Julian calendar. At the time, it was known as the Year of the Consulship of Tiberius and Sejanus (or, less frequently, year 784 Ab urbe condita). The denomination AD 31 for this year has been used since the early medieval period, when the Anno Domini calendar era became the prevalent method in Europe for naming years.

• Lucius Aelius Sejanus is named co-Consul to Emperor Tiberius. However, Tiberius becomes aware of Sejanus' treachery and has him arrested and executed.
• Naevius Sutorius Macro becomes the leader of the Praetorian Guard after Sejanus is executed.

• Gnaeus Arrius Antoninus, Roman consul
• Musonius Rufus, Roman Stoic philosopher (d. 101)

• April 6 – Jesus of Nazareth, founder of Christianity (possible date of the crucifixion)^[1][2][3] (born circa 4 BC) The other possible dates also supported by scholarly consensus among a survey of 100 published scholarly biblical statements are April 7, AD 30 and April 3, AD 33,^[3][4] and April 27, AD 31 (according to a Chodesh calculating system).^[5]
• October 18 – Lucius Aelius Sejanus, Roman prefect and advisor (b. 20 BC)
• Claudia Livia Julia, niece and daughter-in-law of Tiberius (b. 13 BC)^[6]
• Marcus Velleius Paterculus, Roman historian and writer (b. c. 19 BC)
• Nero Julius Caesar Germanicus, grandson and heir of Tiberius (b. AD 6)