|Millennium:||1st millennium BC|
|45 BC by topic|
|Gregorian calendar||45 BC|
|Ab urbe condita||709|
|Ancient Egypt era||XXXIII dynasty, 279|
|- Pharaoh||Cleopatra VII, 7|
|Ancient Greek era||183rd Olympiad, year 4|
|Balinese saka calendar||N/A|
|Chinese calendar||乙亥年 (Wood Pig)|
2652 or 2592
— to —
丙子年 (Fire Rat)
2653 or 2593
|Coptic calendar||−328 – −327|
|Ethiopian calendar||−52 – −51|
|- Vikram Samvat||12–13|
|- Shaka Samvat||N/A|
|- Kali Yuga||3056–3057|
|Iranian calendar||666 BP – 665 BP|
|Islamic calendar||686 BH – 685 BH|
|Julian calendar||45 BC|
|Minguo calendar||1956 before ROC|
|Seleucid era||267/268 AG|
|Thai solar calendar||498–499|
82 or −299 or −1071
— to —
83 or −298 or −1070
Year 45 BC was either a common year starting on Thursday, Friday or Saturday or a leap year starting on Friday or Saturday (link will display the full calendar) (the sources differ, see leap year error for further information) and the first year of the Julian calendar and a leap year starting on Friday of the Proleptic Julian calendar. At the time, it was known as the Year of the Consulship of Caesar without Colleague (or, less frequently, year 709 Ab urbe condita). The denomination 45 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.
- Consuls: Gaius Julius Caesar, without colleague.
- January 1 – The Julian calendar takes effect as the civil calendar of the Roman Empire, establishing a solar calendar that was identical to the Egyptian calendar.
- March 17 – Civil War: In his last victory, Julius Caesar defeats the Pompeian forces of Titus Labienus and Pompey the younger in the Battle of Munda. Labienus dies in battle, Pompey the younger is executed, but Sextus Pompey escapes to take command of the remnants of the Pompeian fleet.
- The veterans of Julius Caesar's Legions Legio XIII Gemina and Legio X Equestris demobilized. The veterans of the 10th legion are settled in Narbo, while those of the 13th are given somewhat better lands in Italia itself.
- Caesar is named dictator for life.
- Caesar probably writes his Commentaries in this year.
- Iullus Antonius, son of Mark Antony and Fulvia; consul 10 BC (d. 2 BC)
- Wang Mang, usurper of the Han Dynasty and Emperor of the Xin Dynasty (d. AD 23)
- February – Tullia, daughter of Cicero (b. 79 BC or 78 BC)
- March 17
- April 12 – Gnaeus Pompeius, son of Pompey the Great, executed after the battle of Munda (b. c. 75 BC)
- December 31 – Quintus Fabius Maximus, consul suffectus 45 BC
- Nigidius Figulus, Roman scholar (b. 98 BC)