|Millennium:||1st millennium BC|
|351 BC by topic|
|Gregorian calendar||351 BC|
|Ab urbe condita||403|
|Ancient Egypt era||XXX dynasty, 30|
|- Pharaoh||Nectanebo II, 10|
|Ancient Greek era||107th Olympiad, year 2|
|Balinese saka calendar||N/A|
|Chinese calendar||己巳年 (Earth Snake)|
2346 or 2286
— to —
庚午年 (Metal Horse)
2347 or 2287
|Coptic calendar||−634 – −633|
|Ethiopian calendar||−358 – −357|
|- Vikram Samvat||−294 – −293|
|- Shaka Samvat||N/A|
|- Kali Yuga||2750–2751|
|Iranian calendar||972 BP – 971 BP|
|Islamic calendar||1002 BH – 1001 BH|
|Minguo calendar||2262 before ROC|
|Thai solar calendar||192–193|
−224 or −605 or −1377
— to —
−223 or −604 or −1376
Year 351 BC was a year of the pre-Julian Roman calendar. At the time it was known as the Year of the Consulship of Peticus and Crispinus (or, less frequently, year 403 Ab urbe condita). The denomination 351 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.
- Encouraged by a failed effort at invasion of Egypt by King Artaxerxes III, Phoenicia and Cyprus revolt against Persia.
- Demosthenes tries to get the Athenians to cease depending on paid mercenaries and return to the old concept of a citizen army. He also delivers his First Philippic, warning Athenians of the folly of believing that Philip's ill health will save Athens from the Macedonians. In response, Athens' citizens vote for increased armaments.
- The Etruscans are badly defeated by the Romans and abandon their attacks on the city and sue for peace.
- First use of the heavy throwing spear, the pilum, (according to Livy) in battle against the Gauls.
- Gaius Marcius Rutilus becomes the first Roman plebeian to be elected to the office of censor.