|Millennium:||1st millennium BC|
|Centuries:||2nd century BC – 1st century BC – 1st century|
|Decades:||70s BC 60s BC 50s BC – 40s BC – 30s BC 20s BC 10s BC|
|Years:||45 BC 44 BC 43 BC – 42 BC – 41 BC 40 BC 39 BC|
|42 BC by topic|
|Gregorian calendar||42 BC
|Ab urbe condita||712|
|Ancient Egypt era||XXXIII dynasty, 282|
|- Pharaoh||Cleopatra VII, 10|
|Ancient Greek era||184th Olympiad, year 3|
|Chinese calendar||戊寅年 (Earth Tiger)
2655 or 2595
— to —
己卯年 (Earth Rabbit)
2656 or 2596
|Coptic calendar||−325 – −324|
|Ethiopian calendar||−49 – −48|
|- Vikram Samvat||15–16|
|- Shaka Samvat||N/A|
|- Kali Yuga||3060–3061|
|Iranian calendar||663 BP – 662 BP|
|Islamic calendar||683 BH – 682 BH|
|Julian calendar||42 BC
|Minguo calendar||1953 before ROC
|Seleucid era||270/271 AG|
|Thai solar calendar||501–502|
|Wikimedia Commons has media related to 42 BC.|
Year 42 BC was either a common year starting on Monday, Tuesday or Wednesday or a leap year starting on Tuesday (link will display the full calendar) of the Julian calendar (the sources differ, see leap year error for further information) and a common year starting on Tuesday of the Proleptic Julian calendar. At the time, it was known as the Year of the Consulship of Lepidus and Plancus (or, less frequently, year 712 Ab urbe condita). The denomination 42 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.
- January – Publius Vatinius, governor of Illyricum, seized Dyrrachium and is forced to surrender his army (three legions) to Marcus Junius Brutus.
- Marcus Brutus begins to plunder the cities of Asia Minor, in order to obtain money and soldiers. The inhabitants of Lycia refused to submit to Rome, and Brutus besieged Xanthus. After destroying their suburbs, the Xanthians withdrew into the heavily fortified city. The Roman legionaries (2,000 men) forced the gate and fight their way into the forum. The citizens made an heroic stand by the temple of Sarpedon, as night falls the Roman army conquers the city. The Xanthians preferred to perish in the flames rather than to yield.
- The confederation of Lycia sent ambassadors to Brutus, promising to form a military league and contribute money for building ships. Gaius Cassius Longinus occupies Rhodes, and ordered all the other cities of Asia to pay a tribute for 10 years.
- July – Mark Antony lands with an army (28 legions) in Illyria, left Octavian ill at Dyrrachium and marched to Amphipolis. Admiral Ahenobarbus blockades with the Republican fleet (130 ships) the Adriatic Sea.
- August – Decidius Saxa and Gaius Norbanus Flaccus are appointed by Antony, to lead an advanced force of eight legions to Macedonia along the Via Egnatia into Thrace.
- September – Brutus and Cassius crossed the Hellespont, they marched to Doriscus but further progress is blocked by Saxa's occupation of the Corpili Pass.
- Saxa retreats to link up with Norbanus at the Sapaei Pass. The Republicans outflank the enemy, forging an alternate route through the mountains in the north.
- Brutus and Cassius advance to Philippi and built fortifications. Antony links up with Norbanus and Saxa at Amphipolis, Octavian arrives on a litter 10 days later.
- Sextus Pompeius leads in Sicily the naval operations in the Mediterranean Sea against the Triumvirs. He blockades the grain routes from Spain and Africa.
- October 3 – First Battle of Philippi: The Triumvirs Mark Antony and Octavian Caesar fight an indecisive battle with Caesar's assassins Marcus Brutus and Cassius. The Roman forces including 2,000 Spartans who just arrived are routed, Octavian takes refuge in the marsh. Cassius' camp is captured by Antony's men, wrongly fearing that Brutus is dead Cassius commits suicide. He ordered his freedman Pindarus to kill him, Brutus feared the impact on morale and secretly buried his beheaded body on Thasos. The Republican navy in the Adriatic, intercept and destroy the supply ships with two legions of the Triumvirs.
- October 23 – Second Battle of Philippi: Brutus' army is defeated by Antony and Octavian, the Triumvirs smash through the weakened Republican centre and take Brutus's right wing in its flank. After the battle 14.000 legionaries lay down their arms. Brutus fled to the heights of Philippi, where he commits suicide the following day. After the victory, Brutus' body is brought to Antonius' camp, where he cast his purple paludamentum over his dead body and orders an honourable funeral for his erstwhile comrade. The Republican cause crushed, Rome rested in the hands of the Second Triumvirate.
- Gaius Julius Caesar Octavian returns to Rome and arranged for ± 40.000 veterans settlements in Campania, Etruria, Picenum, Samnium, Umbria and in northern Italy.
- October 3 – Gaius Cassius Longinus, assassin of Julius Caesar (suicide) (b. bef. 85 BC)
- October 23 – Marcus Junius Brutus, protégé and assassin of Julius Caesar (suicide) (b. 85 BC)
- Marcus Livius Drusus, Roman nobleman and supportor of Brutus (suicide)
- After October 23 – Porcia Catonis, wife of Brutus (suicide) (approximate date) (b. c. 70 BC)
- Gaius Antonius, brother of Mark Antony