|Millennium:||1st millennium BC|
|51 BC by topic|
|Gregorian calendar||51 BC|
|Ab urbe condita||703|
|Ancient Egypt era||XXXIII dynasty, 273|
|- Pharaoh||Cleopatra VII, 1|
|Ancient Greek era||182nd Olympiad, year 2|
|Balinese saka calendar||N/A|
|Chinese calendar||己巳年 (Earth Snake)|
2646 or 2586
— to —
庚午年 (Metal Horse)
2647 or 2587
|Coptic calendar||−334 – −333|
|Ethiopian calendar||−58 – −57|
|- Vikram Samvat||6–7|
|- Shaka Samvat||N/A|
|- Kali Yuga||3050–3051|
|Iranian calendar||672 BP – 671 BP|
|Islamic calendar||693 BH – 692 BH|
|Minguo calendar||1962 before ROC|
|Seleucid era||261/262 AG|
|Thai solar calendar||492–493|
76 or −305 or −1077
— to —
77 or −304 or −1076
Year 51 BC was a year of the pre-Julian Roman calendar. At the time, it was known as the Year of the Consulship of Marcellus and Sulpicius (or, less frequently, year 703 Ab urbe condita). The denomination 51 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: Marcus Claudius Marcellus and Servius Sulpicius Rufus.
- Pompey demands that Julius Caesar lay down his command before he can stand for consul.
- Spring – Pharaoh Ptolemy XII dies and is succeeded by his eldest surviving daughter Cleopatra VII and her younger brother and co-ruler Ptolemy XIII.
- Publius Sulpicius Quirinius, Roman governor of Syria (d. 21 AD)
- Pomponia Caecilia Attica, daughter of Cicero's Epicurean friend Titus Pomponius Atticus.
- Cheng, Emperor of the Han Dynasty of China (d. 7 BC)
- Spring – Pharaoh Ptolemy XII of Egypt (b. 117 BC)
- Posidonius of Apamea, Greek philosopher, astronomer, geographer (b. c. 135 BC)