|Millennium:||1st millennium BC|
|Centuries:||7th century BC – 6th century BC – 5th century BC|
|Decades:||530s BC 520s BC 510s BC – 500s BC – 490s BC 480s BC 470s BC|
|Years:||512 BC 511 BC 510 BC – 509 BC – 508 BC 507 BC 506 BC|
|509 BC by topic|
|Gregorian calendar||509 BC
|Ab urbe condita||245|
|Ancient Egypt era||XXVII dynasty, 17|
|- Pharaoh||Darius I of Persia, 13|
|Ancient Greek era||67th Olympiad, year 4|
|Chinese calendar||辛卯年 (Metal Rabbit)
2188 or 2128
— to —
壬辰年 (Water Dragon)
2189 or 2129
|Coptic calendar||−792 – −791|
|Ethiopian calendar||−516 – −515|
|- Vikram Samvat||−452 – −451|
|- Shaka Samvat||N/A|
|- Kali Yuga||2593–2594|
|Iranian calendar||1130 BP – 1129 BP|
|Islamic calendar||1165 BH – 1164 BH|
|Minguo calendar||2420 before ROC
|Thai solar calendar||34–35|
|Wikimedia Commons has media related to 509 BC.|
The year 509 BC was a year of the pre-Julian Roman calendar. In the Roman Empire it was known as the Year of the Consulship of Brutus and Collatinus (or, less frequently, year 245 Ab urbe condita). The denomination 509 BC 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.
- The Roman monarchy is overthrown, and the Republican period begins.
- The first pair of Roman consuls are elected.
- The Tarquinian conspiracy is formed yet discovered, and the conspirators are executed.
- Forces of Veii and Tarquinii, led by the deposed king Lucius Tarquinius Superbus, are defeated in the Battle of Silva Arsia by the Roman army. Consul Publius Valerius Publicola celebrates the first republican triumph on March 1.
- September 13—The Temple of Jupiter Optimus Maximus on Rome's Capitoline Hill is dedicated on the ides of September.
- Carthage signs a treaty with Rome, delineating their respective spheres.
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- Lucius Junius Brutus, Roman consul and founder of the Roman republic
- Titus Junius Brutus and Tiberius Junius Brutus, brothers (and sons of the consul Lucius Junius Brutus) together with their two uncles the Vitellii and three brothers Aquillii, all executed following the discovery of the Tarquinian conspiracy
- Aruns, son of the last Roman king Lucius Tarquinius Superbus
- Spurius Lucretius Tricipitinus, Roman consul suffectus
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