|Millennium:||1st millennium BC|
|Centuries:||2nd century BC – 1st century BC – 1st century|
|Decades:||100s BC 90s BC 80s BC – 70s BC – 60s BC 50s BC 40s BC|
|Years:||81 BC 80 BC 79 BC – 78 BC – 77 BC 76 BC 75 BC|
|78 BC by topic|
|Gregorian calendar||78 BC|
|Ab urbe condita||676|
|Ancient Egypt era||XXXIII dynasty, 246|
|- Pharaoh||Ptolemy XII Auletes, 3|
|Ancient Greek era||175th Olympiad, year 3|
|Chinese calendar||壬寅年 (Water Tiger)
2619 or 2559
— to —
癸卯年 (Water Rabbit)
2620 or 2560
|Coptic calendar||−361 – −360|
|Ethiopian calendar||−85 – −84|
|- Vikram Samvat||−21 – −20|
|- Shaka Samvat||N/A|
|- Kali Yuga||3024–3025|
|Iranian calendar||699 BP – 698 BP|
|Islamic calendar||720 BH – 719 BH|
|Minguo calendar||1989 before ROC
|Seleucid era||234/235 AG|
|Thai solar calendar||465–466|
|Wikimedia Commons has media related to 78 BC.|
Year 78 BC was a year of the pre-Julian Roman calendar. At the time it was known as the Year of the Consulship of Lepidus and Catulus (or, less frequently, year 676 Ab urbe condita). The denomination 78 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.
- In Rome, Marcus Aemilius Lepidus becomes consul. He attempts to undermine the Sullan reforms and reinstates exiled citizens to reinstall democratic rule.
- The Senate sends Publius Servilius Vatia as proconsul governor to Cilicia, where he fights a campaign against the pirates in southern Anatolia (Lycia, Pamphylia and Isauria).
- The Tabularium is built in the Forum.
- The Third Dalmatian war begins.