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|Centuries:||1st century BC – 1st century – 2nd century|
|Decades:||0s 10s 20s – 30s – 40s 50s 60s|
|Years:||28 29 30 – 31 – 32 33 34|
|31 by topic|
|State leaders – Sovereign states|
|Birth and death categories|
|Births – Deaths|
|Establishment and disestablishment categories|
|Establishments – Disestablishments|
|Ab urbe condita||784|
|Chinese calendar||庚寅年 (Metal Tiger)
2727 or 2667
— to —
辛卯年 (Metal Rabbit)
2728 or 2668
|Coptic calendar||−253 – −252|
|- Vikram Samvat||87–88|
|- Shaka Samvat||N/A|
|- Kali Yuga||3132–3133|
|Iranian calendar||591 BP – 590 BP|
|Islamic calendar||609 BH – 608 BH|
|Minguo calendar||1881 before ROC
|Seleucid era||342/343 AG|
|Thai solar calendar||573–574|
Year 31 (XXXI) was a common year starting on Monday (link will display the full calendar) 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 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 the Emperor Tiberius. However, Tiberius becomes aware of Sejanus' treachery and has him arrested and executed.
- April 6 (Good Friday) – Crucifixion of Jesus (according to one dating scheme).
- Naevius Sutorius Macro becomes the leader of the Praetorian Guard after Sejanus is executed.
- Tiberius returns to Rome from Capri.
- Gnaeus Arrius Antoninus, Roman consul
- Musonius Rufus, (late estimate of birth year) Roman Stoic philosopher (d. 101)
- October 18 – Lucius Aelius Sejanus, Roman politician (executed) (b. 20 BC)
- Marcus Velleius Paterculus, Roman historian (possibly executed as an accomplice of Sejanus) (b. c. 19 BC)
- Livilla, niece and daughter-in-law of the emperor Tiberius (starved to death for her role in a plot to overthrow Tiberius with her lover Sejanus) (b. 13 BC)