|Millennium:||1st millennium BC|
|38 BC by topic|
|Gregorian calendar||38 BC|
|Ab urbe condita||716|
|Ancient Egypt era||XXXIII dynasty, 286|
|- Pharaoh||Cleopatra VII, 14|
|Ancient Greek era||185th Olympiad, year 3|
|Balinese saka calendar||N/A|
|Chinese calendar||壬午年 (Water Horse)|
2659 or 2599
— to —
癸未年 (Water Goat)
2660 or 2600
|Coptic calendar||−321 – −320|
|Ethiopian calendar||−45 – −44|
|- Vikram Samvat||19–20|
|- Shaka Samvat||N/A|
|- Kali Yuga||3063–3064|
|Iranian calendar||659 BP – 658 BP|
|Islamic calendar||679 BH – 678 BH|
|Julian calendar||38 BC|
|Minguo calendar||1949 before ROC|
|Seleucid era||274/275 AG|
|Thai solar calendar||505–506|
89 or −292 or −1064
— to —
90 or −291 or −1063
Year 38 BC was either a common year starting on Sunday or Monday or a leap year starting on Saturday, Sunday or Monday (link will display the full calendar) of the Julian calendar (the sources differ, see leap year error for further information) and a common year starting on Sunday of the Proleptic Julian calendar. At the time, it was known as the Year of the Consulship of Pulcher and Flaccus (or, less frequently, year 716 Ab urbe condita). The denomination 38 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. It was also the first year (year 1) of the Spanish era calendar in use in Hispania until the 15th century.
- January 1 – Beginning of the Hispanic era, by orders of Octavian Caesar.
- January 17 – Octavian marries Livia while she is still pregnant from a recently broken marriage. Octavian gained permission from the College of Pontiffs to wed her while she was still pregnant from another husband. Three months after the wedding she gave birth to a second son, Nero Claudius Drusus, while he and his elder brother, the four-year-old Tiberius, lived in Octavian's household.
- Octavian appointed Marcus Vipsanius Agrippa governor of Transalpine Gaul, where he put down an uprising of the Aquitanians. He also fought successfully against the Germanic tribes, and becoming the next Roman general to cross the Rhine after Julius Caesar.
- Mark Antony, Gaius Julius Caesar Octavianus and Marcus Lepidus sign the Treaty of Tarentum (or 37 BC), extending the Second Triumvirate until 33 BC.
- Parthian invasion into Roman Syria; Publius Ventidius Bassus defeats Pacorus, at the crossing of the Euphrates in the Battle of Cyrrhestica. Ventidius lays siege to Antiochus I Theos of Commagene in Samosata until relieved by Antony.
- January 14 – Nero Claudius Drusus, Roman politician and military commander, future stepson of Augustus Caesar (d. 9 BC)
- Lucius Volusius Saturninus, Roman suffect consul (or 37 BC)