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Year 1421 (MCDXXI) was a common year starting on Wednesday (link will display the full calendar) of the Julian calendar.
|Ab urbe condita||2174|
|Balinese saka calendar||1342–1343|
|English Regnal year||8 Hen. 5 – 9 Hen. 5|
|Chinese calendar||庚子年 (Metal Rat)|
4117 or 4057
— to —
辛丑年 (Metal Ox)
4118 or 4058
|- Vikram Samvat||1477–1478|
|- Shaka Samvat||1342–1343|
|- Kali Yuga||4521–4522|
|Japanese calendar||Ōei 28|
|Minguo calendar||491 before ROC|
|Thai solar calendar||1963–1964|
1547 or 1166 or 394
— to —
1548 or 1167 or 395
- February 2 – Yongle Emperor, third emperor of the Ming Dynasty, shifts the Ming capital from Nanjing to Beijing.
- March 3 – Zheng He receives imperial order from Yongle Emperor to bring imperial letters, silk products, and other gifts to various rulers of countries around the Indian Ocean.
- March 21 – Battle of Baugé: A small French force surprises and defeats a smaller English force under Thomas, Duke of Clarence, a brother of Henry V of England, in Normandy.
- May 26 – Mehmed I, Sultan of the Ottoman Empire, dies and is succeeded by his son, Murad II.
- November 17–19 – St. Elizabeth's flood: The coastal area near Dordrecht in the Netherlands is flooded, due to the extremely high tide of the North Sea; 72 villages are drowned, killing about 10,000 people, and the course of the Meuse is changed.
- John III of Dampierre, Marquis of Namur, sells his estates to Philip the Good, Duke of Burgundy.
- The first patent is issued by the Republic of Florence.
- Portuguese sailors sent by Henry the Navigator cross Cape Non, going as far as Cape Bojador.
- Traditional date – Larabanga Mosque is founded, in modern-day northern Ghana.
- March 9 – Francesco Sassetti, Italian banker (d. 1490)
- May 29 – Charles, Prince of Viana (d. 1461)
- June 3 – Giovanni di Cosimo de' Medici, Italian noble (d. 1463)
- July 25 – Henry Percy, 3rd Earl of Northumberland, English politician (d. 1461)
- August 1 – Thomas Dutton, English knight (d. 1459)
- October 10 – John Paston, English politician (d. 1466)
- December 6 – King Henry VI of England (d. 1471)
- date unknown – Sōgi, Japanese poet and Buddhist priest (d. 1502)
- January 10 – Niccolò I Trinci, lord of Foligno (assassinated)
- January 15 – Helvis of Brunswick-Grubenhagen, queen consort of Armenia and Cyprus (born 1353)
- March 22 – Thomas of Lancaster, 1st Duke of Clarence, second son of Henry IV of England (killed in battle) (born 1388)
- April 21 – John FitzAlan, 13th Earl of Arundel (born 1385)
- May – Balša III, ruler of Zeta
- May 26 – Mehmed I, Ottoman Sultan (b. 1389)
- June 21 – Jean Le Maingre, marshal of France (b. 1366)
- ^ a b Cambridge Antiquarian Society (Cambridge, England) (1958). Proceedings of the Cambridge Antiquarian Society. Deighton Bell. p. 37-38.
- ^ Terence, Kealey (1996), The Economic Laws of Scientific Research
- ^ Szarmach, Paul E.; Tavormina, M. Teresa; Rosenthal, Joel T. (2017). Routledge Revivals: Medieval England (1998): An Encyclopedia. Taylor & Francis. p. 348. ISBN 9781351666374.