38 BC

By place

Roman Republic

• 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 gains permission from the College of Pontiffs to wed her while she is still pregnant from another husband. Three months after the wedding she gives birth to a second son, Nero Claudius Drusus, while he and his elder brother, the four-year-old Tiberius, are living in Octavian's household.
• Octavian appoints Marcus Vipsanius Agrippa governor of Transalpine Gaul, where he puts down an uprising of the Aquitanians. He also fights successfully against the Germanic tribes, and becomes 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, 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.