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.
- 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.
- 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.
- 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.
- Sharafuddin Ali Yazdi's critical history of Persia, Zafar Nama, is completed under the auspices of Mirza Ibrahim Sultan, grandson of Timur.
- 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 besieges La Charité.