|Millennium:||1st millennium BC|
|204 BC by topic|
|Gregorian calendar||204 BC|
|Ab urbe condita||550|
|Ancient Egypt era||XXXIII dynasty, 120|
|- Pharaoh||Ptolemy IV Philopator, 18|
|Ancient Greek era||144th Olympiad (victor)¹|
|Balinese saka calendar||N/A|
|Chinese calendar||丙申年 (Fire Monkey)|
2493 or 2433
— to —
丁酉年 (Fire Rooster)
2494 or 2434
|Coptic calendar||−487 – −486|
|Ethiopian calendar||−211 – −210|
|- Vikram Samvat||−147 – −146|
|- Shaka Samvat||N/A|
|- Kali Yuga||2897–2898|
|Iranian calendar||825 BP – 824 BP|
|Islamic calendar||850 BH – 849 BH|
|Minguo calendar||2115 before ROC|
|Seleucid era||108/109 AG|
|Thai solar calendar||339–340|
−77 or −458 or −1230
— to —
−76 or −457 or −1229
Year 204 BC was a year of the pre-Julian Roman calendar. At the time it was known as the Year of the Consulship of Cethegus and Tuditanus (or, less frequently, year 550 Ab urbe condita). The denomination 204 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.
- Having lost his alliance with the Numidian chief Masinissa, the Carthaginian general, Hasdrubal Gisco, finds a new ally in the Numidian king Syphax, who marries Sophonisba, Hasdrubal's daughter, whom until his defection to Rome has been betrothed to Masinissa.
- Roman forces under Publius Cornelius Scipio besiege Utica in Carthaginia. Scipio is unable to stop the combined forces of the Carthaginians under Hasdrubal Gisco and the Numidians under their chief, Syphax, and he is forced to lift his siege of Utica.
- The late Egyptian King Ptolemy IV's clique of favourites, led by Sosibius, Ptolemy's chief minister, keeps Ptolemy's death a secret, fearing retribution from the new king Ptolemy V's mother, Queen Arsinoe III. They arrange for the murder of Arsinoe, and then the five-year-old king is officially elevated to the throne with Sosibius as his guardian. Arsinoe has been popular with the Egyptian population so rioting follows the news of her assassination.
- The Battle of Crotona is fought between Hannibal's Carthaginian army, and a Roman force led by Publius Sempronius Tuditanus, with no decisive outcome for either side.
- Philip V of Macedon and Antiochus III of the Syrian-based Seleucid kingdom realize Egypt's weakness and agree to partition Egypt's Anatolian and Aegean possessions. Antiochus' share is to be southern Syria, Lycia, Cilicia and Cyprus, while Philip is to gain western Anatolia and the Cyclades.
- Arsinoe III, queen of Egypt, sister and wife of Ptolemy IV (assassinated) (b. c. 246 BC)
- Dong Yi, Chinese general of the Qin Dynasty and conferred with the title of "King of Di"
- Fan Zeng, Chinese adviser during the Chu-Han Contention (206–202 BC) (b. 277 BC)
- Gong Ao, Chinese ruler of the Eighteen Kingdoms during the Chu-Han Contention
- Hanno the Elder, Carthaginian general killed by Scipio Africanus
- Ji Xin, Chinese general during the Chu-Han Contention
- Li Yiji, Chinese politician and adviser (b. 268 BC)
- Long Ju, Chinese general and Grand Marshal
- Ptolemy IV Philopator, king (Pharaoh) of Egypt
- Sima Xin, Chinese general of the Qin Dynasty