The Catilinarian conspiracy, sometimes Second Catilinarian conspiracy, was an attempted coup d'état by Lucius Sergius Catilina (Catiline) to overthrow the Roman consuls of 63 BC – Marcus Tullius Cicero and Gaius Antonius Hybrida – and forcibly assume control of the state in their stead.
The conspiracy was formed after Catiline's defeat in the consular elections for 62, held in early autumn 63. He assembled a coalition of malcontents – aristocrats who had been denied political advancement by the voters, dispossessed farmers, and indebted veterans of Sulla – and planned to seize the consulship from Cicero and Antonius by force. In November 63, Cicero exposed the conspiracy, causing Catiline to flee from Rome and eventually to his army in Etruria. In December, Cicero uncovered nine more conspirators organising for Catiline in the city and, on advice of the senate, had them executed without trial. In early January 62 BC, Antonius defeated Catiline in battle, putting an end to the plot.
Modern views on the conspiracy vary. Uncovering the truth of the conspiracy is difficult. It is well accepted that the ancient sources were heavily biased against Catiline and demonised him in the aftermath of his defeat. The extent of the exaggeration is unclear and still debated. Most classicists agree that the conspiracy occurred as broadly described – rather than being a manipulative invention of Cicero's – but concede that its actual threat to the republic was exaggerated for Cicero's benefit and to heighten later dramatic narratives.