|Millennium:||1st millennium BC|
|55 BC by topic|
|Gregorian calendar||55 BC|
|Ab urbe condita||699|
|Ancient Egypt era||XXXIII dynasty, 269|
|- Pharaoh||Ptolemy XII Auletes, 26|
|Ancient Greek era||181st Olympiad, year 2|
|Balinese saka calendar||N/A|
|Chinese calendar||乙丑年 (Wood Ox)|
2642 or 2582
— to —
丙寅年 (Fire Tiger)
2643 or 2583
|Coptic calendar||−338 – −337|
|Ethiopian calendar||−62 – −61|
|- Vikram Samvat||2–3|
|- Shaka Samvat||N/A|
|- Kali Yuga||3046–3047|
|Iranian calendar||676 BP – 675 BP|
|Islamic calendar||697 BH – 696 BH|
|Minguo calendar||1966 before ROC|
|Seleucid era||257/258 AG|
|Thai solar calendar||488–489|
72 or −309 or −1081
— to —
73 or −308 or −1080
Year 55 BC was a year of the pre-Julian Roman calendar. At the time, it was known as the Year of the Consulship of Crassus and Pompey (or, less frequently, year 699 Ab urbe condita). The denomination 55 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 Licinius Crassus and Gnaeus Pompeius Magnus.
- Consuls Marcus Licinius Crassus and Gnaeus Pompeius Magnus pass the Lex Trebonia.
- Pompey's Theater, the first permanent (non-wooden) theatres in Rome. Built of stone on the Field of Mars, it included a temple to Venus Victorious, a public courtyard, and a meeting hall or curia in the far end near the "Sacred Area".
- Gallic War
- Julius Caesar continues his campaigns in Gaul, he spreads Roman law and order, and makes the whole country as far as the Channel accessible to trade.
- May – Caesar defeats a Germanic army then massacres the women and children, totalling 430,000 people, near the Meuse and Rhine Rivers, now known as the city of Kessel in the Netherlands.
- June – Caesar crosses the Rhine River near the modern site of Koblenz. He constructs a wooden bridge between Andernach and Neuwied (Germany).
- August 22 or August 26 – Julius Caesar commands the first Caesar's invasions of Britain, likely a reconnaissance-in-force expedition, in response to the Britons giving military aid to his Gallic enemies. Caesar retreats back to Gaul when the majority of his force is prevented from landing by storms
- Mithridates III, claimant to the throne of Parthia, supported by Aulus Gabinius, Roman governor of Syria, is defeated by Surena, general under Orodes, in the Battle of Seleucia.
- Berenice IV, queen of Egypt (b. 77 BC)
- Archelaus, high priest of Comana Cappadocia (flourished 1st century BC)
- Lucretius, Roman philosopher (b. c. 99 BC)
- Tigranes the Great, Armenian Emperor (b. c. 140 BC)