The 1420s decade ran from January 1, 1420, to December 31, 1429.
- March – The Çelebi Sultan Mehmed Mosque in Didymoteicho is inaugurated.
- May 21 – Treaty of Troyes: With the Burgundian faction dominant in France, King Charles VI of France acknowledges Henry V of England as his heir, and as virtual ruler of most of France.
- May 25 – Henry the Navigator is appointed governor of the Portuguese Order of Christ.
- June 2 – Catherine of Valois marries King Henry V of England.
- June 7 – Troops of the Republic of Venice capture Udine after a long siege, ending the independence of the Patriarchal State of Friuli, run by the Patriarch of Aquileia.
- August 7 – Construction of the dome of Florence Cathedral is started, after Filippo Brunelleschi wins the commission for his "double shell" design.
- September 1 – a 9.4 MS-strong earthquake shakes Chile's Atacama Region causing tsunamis in Chile as well as Hawaii and Japan.
- October 22 – Ghiyāth al-dīn Naqqāsh, an envoy of the embassy sent by the Timurid ruler of Persia, Mirza Shahrukh (r. 1404–1447), to the Ming Dynasty of China during the reign of the Yongle Emperor (r. 1402–1424), records his sight and travel over a large floating pontoon bridge at Lanzhou (constructed earlier in 1372) as he crosses the Yellow River on this day. He writes that it was: "...composed of twenty three boats, of great excellence and strength attached together by a long chain of iron as thick as a man's thigh, and this was moored on each side to an iron post as thick as a man's waist extending a distance of ten cubits on the land and planted firmly in the ground, the boats being fastened to this chain by means of big hooks. There were placed big wooden planks over the boats so firmly and evenly that all the animals were made to pass over it without difficulty."
- October 28 – Beijing is officially designated the capital of the Ming Dynasty, during the same year that the Forbidden City, the seat of government, is completed.
- November 1 – Hussite Wars – Battle of Vyšehrad: Sigismund, Holy Roman Emperor, fails and is ejected from Bohemia.
- Henry V of England commences construction of the ship Grace Dieu.
- Tang Saier starts a rebellion against the emperor of China, and takes two cities with her rebel army, before she is defeated.
- Construction begins on the Temple of Heaven in Beijing, China while the palace complex of the Forbidden City is completed. In this year the Yongle Emperor confers the title "Beijing" ("Northern Capital") for the Ming Dynasty's new capital city, replacing Nanjing.
- Radu II Praznaglava, supported by the Ottomans, and Dan II, with Hungarian help, start a seven-years-long struggle for the throne of Wallachia, after Mihail I is killed in a battle. The throne of Wallachia will switch from one to another about four times until 1427, when Radu II dies.
- Alexandru cel Bun defends Moldavia against the first incursion by the Ottomans, at Cetatea Albă.
- 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.
- 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.
- January 10 – Hussite Wars – Battle of Deutschbrod: The Hussites defeat 2,000 Royalist Crusaders.
- March 21–May 2 – Hundred Years' War – Siege of Meaux: Meaux surrenders to the English.
- June 10–September – The Ottoman sultan Murad II besieges Constantinople; the siege is broken off as a result of the rebellion of Küçük Mustafa.
- August 22 – Use of the Spanish era dating system in the Kingdom of Portugal ceases.
- September 1 – Henry VI becomes King of England, aged nine months.
- September 27 – The Teutonic Knights sign the Treaty of Melno with the Kingdom of Poland and Grand Duchy of Lithuania, after the brief Gollub War. The Prussian–Lithuanian border established by the treaty remains unchanged, until World War I.
- October 21 – With the death of King Charles VI of France, Henry VI of England is proclaimed King of France in Paris, while the Dauphin, Charles, is proclaimed King Charles VII of France in Bourges.
- Ottoman forces overrun the last domains of Constantine II of Bulgaria, who dies in exile at the Serbian court, ending the Bulgarian Empire.
- On the Italian Peninsula, Venice has a population of 84,000, of which 200 men rule the city, while Florence has a population of 40,000, of which 600 men rule the city.
- April 27 – Hussite Wars – Battle of Hořice: The Taborites decisively beat the Utraquists.
- May 21–22 – Byzantine–Ottoman Wars: The Ottoman governor of Thessaly, Turakhan Beg, breaks through the Hexamilion wall, and ravages the Peloponnese Peninsula.
- July 31 – Hundred Years' War – Battle of Cravant: The French army is defeated at Cravant, on the banks of the River Yonne near Auxerre, by the English and their Burgundian allies.
- August – The Treaty of Sveti Srdj ends the Second Scutari War, waged between the Serbian Despotate and the Venetian Republic, over Scutari, and other former possessions of Zeta, captured by the Venetians.
- The three independent boroughs of Pamplona are united into a single town by royal decree, after centuries of feuds.
- Dan II of Wallachia, with Hungarian help, wins two battles against the Ottomans.
- June 2 – Battle of L'Aquila: Jacopo Caldora and Micheletto Attendolo, for the Kingdom of Naples, defeat Braccio da Montone, for Alfonso V of Aragon.
- August 17 – Battle of Verneuil: An English force under John, Duke of Bedford defeats a larger French army under the Duke of Alençon, John Stewart, and Earl Archibald of Douglas. Alençon is captured and Douglas killed.
- Dalmatia: Aliota Capenna, lord of Lesina (nowadays Hvar), offers his realm to the Republic of Venice (also said to have occurred in 1409 and 1421).
- November 9 – Kale Kye-Taung Nyo becomes King of Ava by having his lover, Queen Shin Bo-Me, assassinate his 8-year-old nephew, King Min Hla.
- December 9 – The Old University of Leuven, Belgium is founded.
- Date unknown
- The Maltese people rise up against Don Gonsalvo Monroy, count of Malta. The insurgents repel an attempt by the Viceroy of Sicily to bring the island to order. The Maltese do not submit to Catalan-Aragonese rule, until the Magna Charta Libertatis, granting them their new rights, is delivered to them.
- Beijing, capital of China, becomes the largest city in the world, taking the lead from Nanjing (estimated date).
- By this year, paper currency in China is worth only 0.025% to 0.014% of its original value in the 14th century; this, and the counterfeiting of copper coin currency, will lead to a dramatic shift to using silver as the common medium of exchange in China.
- Sharafuddin Ali Yazdi's critical history of Persia, Zafar Nama, is completed under the auspices of Mirza Ibrahim Sultan, grandson of Timur.
- March 6 – Battle of St. James (near Avranches): An English army under John, Duke of Bedford defeats the French under Arthur de Richemont, forcing the Duke of Brittany to recognize English suzerainty.
- c. May 15 - 16 – Kale Kye-Taung Nyo, ruler of the Kingdom of Ava, flees his capital. He is succeeded by Mohnyin Thado, who receives Thinkhaya III of Toungoo.
- June 16 – Hussite Wars – Battle of Usti nad Labem: The Hussites decisively beat the crusading armies in the Fourth Anti-Hussite Crusade.
- July 7 – Battle of Chirokitia: King Janus of Cyprus is defeated and captured by the Mamluks and brought to Egypt, where he is ransomed after ten months.
- Date unknown
- January – Spring – Radu II of Wallachia resumes the throne for the fourth time, but a seven-year struggle for it ends when he is defeated in battle, and probably killed, by Dan II, who resumes the throne for a fifth term.
- April – The House of Balsic's rule of Montenegro comes to an end with the death of Balša III.
- August 4 – Hussite Wars: Battle of Tachov – The Hussites decisively beat the crusader armies, ending the Fourth Anti-Hussite Crusade.
- August 17 – The first band of Gypsies visits Paris, according to an account of the citizens of Paris.
- October 13 – Lincoln College, a constituent college of the University of Oxford in England, is founded by the Bishop of Lincoln.
- Gabriel V is elected Patriarch of the Coptic Church for the second time.
- Minrekyansa becomes King of Ava (ancient Burma).
- The Conflict of Druimnacour occurs in Sutherland, Scotland.
- The first witch hunts begin, in Switzerland.
- The Celestine Order is established in France.
- The Celebration of Sant Jordi (Saint George) begins in Catalonia (he will later become its patron saint).
- Bremen is expelled from the Hanseatic League.
- Diogo de Silves, Portuguese navigator, discovers seven islands of the Azores archipelago.
- Battle of the Echinades: A Byzantine fleet defeats the fleet of Carlo I Tocco.
- Itzcoatl becomes the 4th Tlatoani of Tenochtitlan, after his nephew Chimalpopoca is killed by the Maxtla, at Azcapotzalco.
- Bhaktapur Royal Palace (in Nepal) is built by King Yaksa Malla.
- June 3 – Dan II leads an army against the Ottomans at Golubac Fortress, obtaining a treaty that will allow him a semi-peaceful rule in Wallachia, until 1432.
- August 30 – Emperor Go-Hanazono accedes to the throne of Japan.
- October 12 – English forces under Thomas Montacute, 4th Earl of Salisbury, besiege Orléans. Jean de Dunois, the Bastard of Orléans, commands the defenders.
- October 24 – Thomas Montacute, 4th Earl of Salisbury, is mortally wounded in an unsuccessful assault on Orléans. He is succeeded in command by William de la Pole, 4th Earl of Suffolk.
- Itzcóatl becomes ruler of the Aztecs. He eventually begins the construction of Tenochtitlan.
- The Aztec Triple Alliance (also known as The Aztec Empire) forms with the alliance of three Aztec city-states—Tenochtitlán, Texcoco, and Tlacopán—and defeats Azcapotzalco to win control of the Valley of Mexico.
- The Valais witch trials begin. 
- A serious fire occurs at Baynard's Castle in London, England.
- Voices tell Joan of Arc that Charles VII of France must be crowned, and the English expelled from France.
- Lam Sơn uprising: Lê Lợi, founder of the Lê Dynasty in Vietnam, liberates Annam (the territory occupied by Ming Dynasty China in 1407), and restores the kingdom as Đại Việt.
- February 12 – Battle of Rouvray (or "of the Herrings"): English forces under Sir John Fastolf defend a supply convoy, which is carrying rations (food) to the army of William de la Pole, 4th Earl of Suffolk at Orléans, from attack by the Comte de Clermont and John Stewart.
- April 29 – Joan of Arc arrives to relieve the Siege of Orléans.
- April 29 – Siege of Orléans: Joan of Arc enters Orléans with a relief expedition.
- May 7 – The Tourelles, the last English siege fortification at Orléans, falls. Joan of Arc becomes the hero of the battle by returning, wounded, to lead the final charge.
- May 8 – The English, weakened by disease and lack of supplies, depart Orléans.
- June 18 – Battle of Patay: French forces under Joan of Arc smash the English forces under Lord Talbot and Sir John Fastolf, forcing the withdrawal of the English from the Loire Valley.
- July 17 – Charles VII of France is crowned in Rheims.
- September – The Hafsid Saracens attempt to capture Malta, but are repelled by its defenders.
- September 8 – Joan of Arc leads an unsuccessful attack on Paris, and is wounded.
- November 4 – Armagnac–Burgundian Civil War: Joan of Arc liberates Saint-Pierre-le-Moûtier.
- November 24 – Joan of Arc besieges La Charité.
Joan of Arc
- Guzmán, L. (February 14, 2019). "Encuentran registros de megaterremoto ocurrido hace seis siglos en el norte de Chile". El Mercurio (in Spanish). Santiago, Chile. Retrieved June 8, 2019.
- Manuel Abad, Tatiana Izquierdo, Miguel Cáceres, Enrique Bernárdez and Joaquín Rodríguez‐Vidal (2018). Coastal boulder deposit as evidence of an ocean‐wide prehistoric tsunami originated on the Atacama Desert coast (northern Chile). Sedimentology. Publication: december, 13th, 2018. https://doi.org/10.1111/sed.12570
- Lily Xiao Hong Lee, Sue Wiles: Biographical Dictionary of Chinese Women, Volume II: Tang Through Ming 618 - 1644
- 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.
- Babinger, Franz (1993) [1913–1936]. "Turakhān Beg". In Houtsma, Martijn Theodoor (ed.). E.J. Brill's First Encyclopaedia of Islam. VIII. Leiden: Brill. pp. 876–878. ISBN 90-04-09796-1.
- Stephanopoli, Dimo (1799). Voyage de Dimo et Nicolo Stephanopoli en Grèce,: pendant les années V et VI. Paris: Guilleminet.
- "Geography". about.com.
- Gernet, Jacques (translated by Foster, J. R. and Hartman, Charles) (1936). A History of Chinese Civilization. Cambridge University Press. p. 407.
- The History of the Feuds and Conflicts Among the Clans in the Northern Parts of Scotland and in the Western Isles: from the year M.XX1 unto M.B.C.XIX, now first published from a manuscript wrote in the reign of King James VI. Foulis press, 1764.
- Burns, William E. (2003). Witch Hunts in Europe and America: An Encyclopedia. Greenwood Publishing Group. p. 296. ISBN 9780313321429.
- Richardson, John (2000). The Annals of London: A Year-by-year Record of a Thousand Years of History. University of California Press. p. 63. ISBN 9780520227958.
- "Saint Joan of Arc | French heroine". Encyclopedia Britannica. Retrieved 6 May 2019.