Please note that dates in 1821 follow the OS calendar, out of necessity, as they revolve around the 25 March (OS) declaration of Independence.
- March 6: Alexander Ypsilantis crosses the Prut River with Olympios Georgakis and others and starts the Greek Revolution in Romania.
- March 17: Petrobey Mavromichalis declares revolution against the Turks in Areopolis.
- March 23: Kalamata is liberated two days before the Greek War of Independence is declared.
- March 25: Greece declares the War of Independence against Turkey.
- March 26: Aigion is liberated during the Greek War of Independence
- March 28: Three days after the proclamation of the Greek Revolution, a provisional government is formed in the liberated city of Kalamata.
- March 29: Livadeia joins the War of Independence under Athanasios Diakos.
- April 3: Spetses joins the Greek War of Independence.
- April 10: Patriarch Gregory V is hanged on the Central outside porte of the Patriarchate by the Turks.
- April 10: The island of Psara joins the Greek War of Independence.
- April 10: Amfissa is liberated during the Greek War of Independence.
- April 16: Hydra joins the Greek War of Independence.
- April 22: The Battle of Alamana takes place during the Greek War of Independence.
- April 24: Athanasios Diakos is executed by the Turks.
- April 26: During the Greek War of Independence, Athens is liberated by Greek forces under Dimos Antoniou.
- May 2: The Turks capture Galati, Moldavia.
- May 3: A large Turkish force is sent to Cyprus to prevent Cypriots from joining the Greek War of Independence.
- May 8: The Battle of the inn of Gravia is fought.
- May 8: Samos, under Lycurgus Logothetis, joins the Greek War of Independence.
- May 10: Andros joins the Greek War of Independence.
- May 12: The Battle of Valtetsi commences.
- May 13: The Battle of Valtetsi ends in Greek victory.
- May 18: Greek revolutionaries, under Nikitaras, defeat the Turkish army at the Battle of Doliana.
- May 27: Dimitris Papanikolis and his companions Yiannis Patatoukos and Giorgos Kalafatis burn a 76-cannon Turkish war ship in Eressos harbour, Lesbos.
- June 2: Ayvali is destroyed by Turkish troops.
- June 11: Agrinio is liberated during the Greek War of Independence.
- June 14: The Greek War of Independence breaks out in Crete with an attack against Turkish positions near Loulo, Chania.
- June 19: The Battle of Dragasani takes place.
- June 19: Patriarch Gregory V is buried in Odessa.
- June 23: The Turks execute Archbishop Gerasimos of Crete in retaliation for the breakout of the Greek War of Independence on the island.
- July 9: Archbishop Kyprianos of Cyprus is hanged by the Turks in an attempt to intimidate the Cypriots from joining the Greek Revolution. The bishops of Paphos, Kition and Kyrenia are also executed.
- July 28: The Souliotes are forced to abandon Souli after Ali Pasha is defeated by the Sultan's forces.
- August 1: The first Greek newspaper, the "Greek Herald", is published in Kalamata.
- August 1: Monemvasia is liberated during the Greek Revolution.
- November 4: The revolting Greeks prevail over the Turkish forces at Chalandri and then take over Athens
- November 5: First lord high commissioner of the Ionian Islands, Sir Thomas Maitland, expels all Greek refugees to prevent residents of the islands from giving assistance to the Greek War of Independence.
- November 13: Greek revolutionaries take Arta during the War of Independence.
- November 20: The First National Assembly of Independence begins at Epidaurus (it will conclude on January 15, 1822)