|Centuries:||1st century BC – 1st century – 2nd century|
|Decades:||10s 20s 30s – 40s – 50s 60s 70s|
|Years:||39 40 41 – 42 – 43 44 45|
|42 by topic|
|State leaders – Sovereign states|
|Birth and death categories|
|Births – Deaths|
|Establishment and disestablishment categories|
|Establishments – Disestablishments|
|Ab urbe condita||795|
|Bahá'í calendar||−1802 – −1801|
|English Regnal year||N/A|
|Chinese calendar||辛丑年 (Metal Ox)
2738 or 2678
— to —
壬寅年 (Water Tiger)
2739 or 2679
|Coptic calendar||−242 – −241|
|- Vikram Samvat||98–99|
|- Shaka Samvat||N/A|
|- Kali Yuga||3143–3144|
|Igbo calendar||−958 – −957|
|Iranian calendar||580 BP – 579 BP|
|Islamic calendar||598 BH – 597 BH|
|Minguo calendar||1870 before ROC
|Thai solar calendar||585|
|Wikimedia Commons has media related to 42.|
Year 42 (XLII) was a common year starting on Monday (link will display the full calendar) of the Julian calendar. At the time, it was known as the Year of the Consulship of Caesar and Piso (or, less frequently, year 795 Ab urbe condita). The denomination 42 for this year has been used since the Early Middle Ages, when the Anno Domini calendar era became the prevalent method in Europe for naming years.
- Consuls are Emperor Claudius and Gaius Caecina Largus.
- Romans take control of Ceuta, a port city on the North African side of the Strait of Gibraltar.
- The territories of the current Algeria and Morocco become a Roman province.
- The Dalmatian legate, Furius Camillus Scribonianus, revolts but his troops defect and his rebellion quickly withers.
- Claudius begins construction of Portus, the harbour bears a lighthouse and is sited on the right bank of the Tiber.
- Suro becomes the first king of Geumgwan Gaya in the Korean Peninsula.
- The Chinese General Ma Yuan represses the rebellions of the Trưng Sisters in Tonkin.
- 25 January – Conversion of the Apostle Paul to Christianity. The exact date is not provided in texts, but the Roman Catholic Church has chosen to commemorate this date.