The 1020s was a decade of the Julian Calendar which began on January 1, 1020, and ended on December 31, 1029.
- Summer – Emperor Henry II conducts his third Italian military campaign. He makes plans to invade the south, but remains non-commital.
- June 15 – Byzantine troops under Catepan Basil Boioannes (supported by his ally Prince Pandulf IV) capture the fortress of Troia.
- The French city of Saint-Germain-en-Laye is founded by King Robert II (the Pious).
- King Canute the Great codifies the laws of England (approximate date).
- King Gagik I of Armenia is succeeded by Hovhannes-Smbat III
- November – Emperor Henry II conducts his fourth Italian military campaign. He crosses the Brenner Pass with a 60,000-strong army and reaches Verona, where he receives Lombard levies. Henry proceeds to Mantua and then into Ravenna, to spend the Christmas.
- The Taifa of Valencia, a Moorish kingdom in Al-Andalus (modern Spain), becomes independent from the Caliphate of Córdoba (approximate date).
- Senekerim-Hovhannes Artsruni, king of Vaspurakan (Greater Armenia), surrenders his kingdom to the Byzantine Empire. He receives in return Sebasteia and becomes governor of Cappadocia.
- Hovhannes-Smbat III, King of the Armenian kingdom of Ani was attacked by his younger brother Ashot IV, and lost much of his power to him, becoming concurrent king of outlying territories.
- The Chinese capital city of Kaifeng has some half a million residents by this year; including all those present in the nine designated suburbs, the population is over a million people.
- Emperor Rajendra Chola I extends his influence of the Chola Empire to the banks of the Ganges River (North India) and invades Bengal.
- Sultan Mahmud of Ghazni appoints Malik Ayaz to the throne, making Lahore (modern Pakistan) the capital of the Ghaznavid Empire.
- Spring – Battle of Svindax: The Byzantine army under Emperor Basil II defeats the Georgians at Svindax (modern Turkey). King George I is forced to negotiate a peace treaty, ending the Byzantine–Georgian wars.
- Summer – Nikephoros Phokas (Barytrachelos) conspires with the Byzantine general Nikephoros Xiphias against Basil II. The rebellion collapses and Xiphias assassinates Phokas.
- Spring – Emperor Henry II divides his army into three columns and descends through Rome onto Capua. The bulk of the expeditionary force (20,000 men) led by Henry, makes its way down the Adriatic coast.
- Pilgrim, archbishop of Cologne, marches with his army down the Tyrrhenian coast to lay siege to Capua. The citizens open the gates and surrender the city to the imperial army.
- Pilgrim besieges the city of Salerno for forty days. Prince Guaimar III offers to give hostages – Pilgrim accepts the prince's son and co-prince Guaimar IV, and lifts the siege.
- Summer – Outbreak of the plague among the German troops forces Henry II to abandon his campaign in Italy. He reimposes his suzerainty on the Lombard principalities.
- King Olof Skötkonung dies and is succeeded by his son Anund Jakob (or James) as ruler of Sweden. He becomes the second Christian king of the Swedish realm.
- The 14-year-old Al-Mu'izz ibn Badis takes with support of the Zirid nobles the government over and ascends (as a minor) to the throne in Ifriqiya (modern Tunisia).
- The Chinese military has one million registered soldiers during the Song Dynasty, an increase since the turn of the 11th century (approximate date).
- King Robert II (the Pious) burns 13 Cathari heretics at Orléans. These are the first burning victims for heresy in Medieval Europe.
- Pope Benedict VIII convenes a synod at Pavia. He issues decrees to restrain simony and incontinence of the clergy.
- Æthelnoth, archbishop of Canterbury, travels to Rome to obtain the pallium, which is received by Benedict VIII.
- The Judge-Governor of Seville in Al-Andalus (modern Spain) takes advantage of the disintegration of the Caliphate of Córdoba, and seizes power as Abbad I, founding the Abbadid Dynasty.
- December – Abbad I declares the Taifa of Seville independent from Córdoban rule. Abd ar-Rahman V is proclaimed Caliph at Córdoba.
- The Dom Church at Utrecht (modern Netherlands) is severely damaged by fire. Bishop Adalbold II builds a new Romanesque style church.
- Emperor Basil II prepares a Byzantine expedition to invade Sicily. Governor Ahmed al-Akhal appeals to the Zirids of Ifriqiya for help. They dispatch a fleet, but these is caught up in a storm and destroyed near Pantelleria.
- Battle of Lemnos: Kievan Viking raiders (800 men) sail through the straits at Abydos to the Aegean Sea. From there they made for the island of Lemnos, but are defeated by a Byzantine fleet of the Cibyrrhaeot Theme.
- July 13 – Emperor Henry II dies in his imperial palace at Göttingen (modern Germany). He leaves no heirs, thereby ending the Ottonian Dynasty. The Salian Dynasty of the Holy Roman Empire is founded by Conrad II.
- September – Conrad II (the Elder) is elected and crowned as King of Germany in Mainz, while both he and his cousin Conrad the Younger (son of Conrad I, duke of Carinthia) are invested as joint dukes of Franconia.
- Roger I of Tosny, a Norman nobleman, leaves the battlefield of the Ebro Valley after terrorising the Saracens, and capturing several towns and castles during the Reconquista in the Iberian Peninsula (modern Spain).
- The world's first paper-printed money, which later greatly benefits the economy of the Song Dynasty, originates in the Sichuan province of China.
- Sultan Mahmud of Ghazni sacks the Hindu religious center of Somnath, and takes away a booty of 20 million dinars (approximate date).
- April 9 – Pope Benedict VIII dies after a 12-year pontificate at Rome. He is succeeded by his brother John XIX as the 144th pope of the Catholic Church.
- December 15 – Emperor Basil II (Bulgar Slayer) dies in Constantinople after a 50-year reign. Never married – he is succeeded by his brother Constantine VIII who becomes sole ruler of the Byzantine Empire. Constantine calls the Sicilian invasion off. Catapan Basil Boioannes diverts the Byzantine expeditionary force already assembled on Calabria to join the siege of Capua.
- April 18 – Bolesław I (the Brave) is crowned in Gniezno as the first king of Poland. He takes advantage of the interregnum in Germany (see 1024), and receives permission for his coronation from Pope John XIX.
- September – At the urging of Queen Constance of Arles, the three sons of King Robert II (the Pious) revolt against their father – Hugh Magnus (heir and co-king), Henry I and Robert I start a civil war over power.
- December 25 – Mieszko II, son of Bolosław I, is crowned as king of Poland by Archbishop Hippolytus in the Gniezno Cathedral.
- Emir Al-Mu'izz ibn Badis of the Zirid Dynasty in Ifriqiya (modern Tunisia) attempts to retake Sicily but fails.
- Srivijaya, a Buddhist kingdom based in Sumatra, is attacked by Emperor Rajendra I of the Chola Empire of southern India, in a dispute over trading rights in Southeast Asia. It survives, but declines in importance.
- Spring – King Conrad II (the Elder) assembles an army of thousands of armored knights for an expedition into Italy. He besieges Pavia and marches to Milan, where he is crowned with the Iron Crown by Archbishop Aribert as king of the Lombards. Duke William V (the Great) of Aquitaine, who is already en route for Italy, decides to renounce his claim to the Lombard throne and turns back.
- April – Conrad II punishes (with the help of Milanese troops) the citizens of Pavia with starvation, for burning down the Royal Palace. He appoints Aribert as his viceroy ("imperial vicar") in Italy and charges him with ensuring that the order is complied.
- Summer – Conrad II leaves the bulk of his army at the siege of Pavia and marches to Ravenna. The Ravennan militias close the town gates and assault the imperial train. Conrad rallies his troops and takes Ravenna, taking bloody revenge.
- Conrad II proceeds to Pesaro, but a malarian outbreak forces him to withdraw back up north to the Po Valley. He subdues the March of Turin, where Count Ulric Manfred II opposes the election of Conrad.
- Battle of the Helgeå (off the coast of Sweden): Naval forces of King Cnut the Great's North Sea Empire defeat the combined Swedish and Norwegian royal fleets.
- Autumn – Pavia falls to the imperial forces. Only the intervention of Odilo of Cluny persuades Conrad to have mercy on the city and the defeated rebels.
- The 9-year-old Henry VI (the Black) is made duke of Bavaria by his father, Conrad II. After the death of his predecessor Henry V.
- Pietro Barbolano becomes the 28th doge of Venice.
- A Zubu revolt against the Liao Dynasty is suppressed, with the Zubu forced to pay an annual tribute of horses, camels and furs.
- March 26 – Pope John XIX crowns Conrad II (the Elder) and his wife Gisela as Holy Roman Emperor and Empress, respectively, in Old St. Peter's Basilica in Rome.
- May 14 – King Robert II (the Pious) sues for peace with his sons. Henry I is crowned co-king of France at Reims Cathedral, but has little power to rule (until 1031).
- Duke Sergius IV of Naples donates the County of Aversa to a band of Norman mercenaries led by Rainulf Drengot, who support him in the war with Capua.
- August 6 – Robert I (the Magnificent) becomes duke of Normandy after the death of his brother Richard III.
- King Cnut the Great attends the coronation of Conrad II in Rome. This proves his position as sole ruler of the Danish North Sea Empire.
- King Sitric Silkbeard of Dublin and sub-King Flannacán of Brega make a pilgrimage to Rome.
- Ealdred is appointed abbot of Tavistock Abbey (approximate date).
- August 16 – Bagrat IV becomes king of Georgia and succeeds his father, George I, after his death. Queen Dowager Mariam becomes regent for her underage son.
- As recorded in the Song Shi, the Song Dynasty Chinese engineer Yan Su reinvents the 3rd-century south-pointing chariot, a mechanical-driven compass vehicle.
- This is the first year of the first rabjyung (60-year) cycle started in the Tibetan calendar.
- The Book of Healing (Arabic: کتاب الشفاء Kitab Al-Shifaʾ, Latin: Sufficientia), a comprehensive scientific and philosophical encyclopedia written by the Persian polymath Avicenna (Abū ʿAlī ibn Sīnā), is published.
- November 11 – Emperor Constantine VIII dies at Constantinople after a 3-year reign. On his deathbed, and without a male heir, Constantine arranges that his eldest daughter, Zoë Porphyrogenita, succeeds him and marries the Byzantine nobleman, Romanos III (Argyros).
- November 15 – Zoë Porphyrogenita takes the throne as empress consort. Her husband, Romanos III (age 60) becomes emperor of the Byzantine Empire.
- Cnut the Great sails from England to Norway with a fleet of 50 ships. He defeats Olaf Haraldsson and is crowned king of Norway. Cnut becomes the sole ruler of England, Denmark and part of Sweden (known as the Danish North Sea Empire).
- April 14 – The 10-year-old Henry III (the Black), son of Emperor Conrad II (the Elder), is elected and crowned king of Germany in Aachen Cathedral by Pilgrim, archbishop of Cologne.
- King Sancho Garcés III (the Great) conquers Castile (modern Spain) (approximate date).
- Prince Pandulf IV of Capua becomes the de facto ruler of southern Italy – holding Capua and Naples himself – this in support with his powerful allies Amalfi, Salerno and Benevento. Only the Duchy of Gaeta remains out of his grasp.
- Rainulf Drengot, head of a mercenary band of Norman knights, is approached by Duke John V of Gaeta and is persuaded to change sides. With Norman help, Duke Sergius IV recovers Naples from Capuan occupation.
- Duke Bretislav I (Bohemian Achilles) of Bohemia of the Přemyslid Dynasty reconquers Moravia from Poland (approximate date).
- Bresc, Henri (2003). "La Sicile et l'espace libyen au Moyen Age" (PDF). Retrieved 17 January 2012.
- Hewsen, Robert H. (2001). Armenia: A Historical Atlas. The University of Chicago Press. p. 126. ISBN 0-226-33228-4.
- Norwich, John Julius (1967). The Normans in the South. London: Longman, pp. 26–28.
- Amatus, Dunbar & Loud (2004), p. 53. The young prince was sent to the papal court for safekeeping according to Amatus.
- Walker, Williston (1921). A History of the Christian Church. Charles Scribner's Sons, p. 218.
- Ortenberg. Anglo-Saxon Church and the Papacy. English Church and the Papacy, p. 49.
- Wortley, John ed. (2010). John Skylitzes: A Synopsis of Byzantine History, 811–1057. Cambridge: Cambridge University Press. ISBN 978-0-521-76705-7.
- Boissonade, B. "Les premières croisades françaises en Espagne. Normands, Gascons, Aquitains et Bourguignons (1018-1032)". Bulletin Hispanique. 36 (1): 5–28. doi:10.3406/hispa.1934.2607.
- Gilbert Meynier (2010) L'Algérie cœur du Maghreb classique. De l'ouverture islamo-arabe au repli (658-1518). Paris: La Découverte; p.50.
- Jonathan Riley-Smith (2004). The New Cambridge Medieval History. Volume IV c. 1024–c. 1198, p. 72. ISBN 978-0-521-41411-1.
- Dated 1025 by the Anglo-Saxon Chronicle, which gives the victory to Sweden.
- Lucy Margaret Smith (1920). The Early History of the Monastery of Cluny. Oxford University Press.
- Herwig Wolfram, Conrad II, 990-1039: Emperor of Three Kingdoms, p. 102.
- William W. Clark, Medieval Cathedrals, (Greenwood Publishing, 2006), p. 87.
- Goodman, Lenn Evan (1992). Avicenna. London: Routledge. p. 31. ISBN 0-415-01929-X.