|Millennium:||1st millennium BC|
|406 BC by topic|
|Gregorian calendar||406 BC|
|Ab urbe condita||348|
|Ancient Egypt era||XXVII dynasty, 120|
|- Pharaoh||Darius II of Persia, 18|
|Ancient Greek era||93rd Olympiad, year 3|
|Balinese saka calendar||N/A|
|Chinese calendar||甲戌年 (Wood Dog)|
2291 or 2231
— to —
乙亥年 (Wood Pig)
2292 or 2232
|Coptic calendar||−689 – −688|
|Ethiopian calendar||−413 – −412|
|- Vikram Samvat||−349 – −348|
|- Shaka Samvat||N/A|
|- Kali Yuga||2695–2696|
|Iranian calendar||1027 BP – 1026 BP|
|Islamic calendar||1059 BH – 1058 BH|
|Minguo calendar||2317 before ROC|
|Thai solar calendar||137–138|
−279 or −660 or −1432
— to —
−278 or −659 or −1431
Year 406 BC was a year of the pre-Julian Roman calendar. At the time, it was known as the Year of the Tribunate of Cossus, Ambustus, Cossus and Potitus (or, less frequently, year 348 Ab urbe condita). The denomination 406 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.
- Callicratidas is appointed as the navarch of the Spartan fleet, replacing Lysander. Callicratidas assembles a fleet and sails to Methymna, on Lesbos, to which he lays siege. This move threatens the Athenian grain supply.
- Alcibiades is replaced by a board of generals. Athens sends a member of the board, Admiral Conon, to relieve the siege of Mytilene. To defend Lesbos, Conon is forced to move his numerically inferior fleet from Samos to the Hekatonnesi islands near Methymna. When Callicratidas attacks him, Conon is forced back to Mytilene, where he is blockaded by Callicratidas' Spartan fleet.
- Athens wins the Battle of Arginusae, near Lesbos, and the blockade of Conon is broken. To relieve Conon, the Athenians assemble a new fleet composed largely of newly constructed ships crewed by inexperienced sailors. This inexperienced fleet is inferior to the Spartans, but its commanders employ new and unorthodox tactics, which allow the Athenians to secure a dramatic and unexpected victory. The Spartan force is soundly defeated, and Callicratidas is killed.
- Returning to Athens after the battle, Theramenes leads Athenian agitation against the eight generals who have commanded in the engagement; the six who have returned to Athens are condemned for negligence in not having picked up survivors from the ships disabled in the battle. The Athenian generals (including Pericles' son) are put to death.
- Sparta sues for peace, which the Athenian leader Cleophon rejects. Sparta yields to demands by the Persian satrap Cyrus that Lysander command a fleet in the Hellespont.
- The Carthaginians again invade Sicily and attack Agrigentum (Acragas). Plague breaks out in their camp and Hannibal Mago dies. Himilco assumes command and captures Agrigentum (Acragas), Gela and Camarina. Gela is destroyed and its treasures sacked. The survivors take refuge in Syracuse. The plague is carried back to Carthage by its soldiers.
- Euripides, Athenian playwright (b. c. 480 BC)
- Hannibal Mago, Carthaginian general
- Sophocles, Athenian dramatist and politician (b. c. 495 BC)