| 420 BC by topic | |
| Politics | |
| State leaders – Sovereign states | |
| Birth and death categories | |
| Births – Deaths | |
| Establishments and disestablishments categories | |
| Establishments – Disestablishments | |
| Gregorian calendar | 420 BC |
| Ab urbe condita | 334 |
| Armenian calendar | N/A |
| Assyrian calendar | 4331 |
| Bahá'í calendar | -2263–-2262 |
| Bengali calendar | -1012 |
| Berber calendar | 531 |
| English Regnal year | N/A |
| Buddhist calendar | 125 |
| Burmese calendar | -1057 |
| Byzantine calendar | 5089–5090 |
| Chinese calendar | 庚申年 (2217/2277) — to —
辛酉年(2218/2278) |
| Coptic calendar | -703–-702 |
| Ethiopian calendar | -427–-426 |
| Hebrew calendar | 3341–3342 |
| Hindu calendars | |
| - Vikram Samvat | -363–-362 |
| - Shaka Samvat | N/A |
| - Kali Yuga | 2682–2683 |
| Holocene calendar | 9581 |
| Iranian calendar | 1041 BP – 1040 BP |
| Islamic calendar | 1073 BH – 1072 BH |
| Japanese calendar | |
| Korean calendar | 1914 |
| Minguo calendar | 2331 before ROC 民前2331年 |
| Thai solar calendar | 124 |
Year 420 BC was a year of the pre-Julian Roman calendar. At the time, it was known as the Year of the Tribunate of Cincinnatus, Vulso, Medullinus and Atratinus (or, less frequently, year 334 Ab urbe condita). The denomination 420 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.