|Millennium:||1st millennium 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|
|Balinese saka calendar||N/A|
|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||3023–3024|
|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|
49 or −332 or −1104
— to —
50 or −331 or −1103
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. Later and less frequently, it was known as the year 676 AUC). 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, quarrels with his co-consul, is sent to govern Transalpine Gaul, and initiates a rebellion against the Senate with his army there.
- The Senate sends Publius Servilius Vatia as proconsul governor to Cilicia, where he fights a campaign against the Piracy in southern Anatolia (Lycia, Pamphylia and Isauria).
- The Tabularium is built in the Forum.
- The Third Dalmatian war begins.