1643 (MDCXLIII) was a common year starting on Thursday of the Gregorian calendar and a common year starting on Sunday of the Julian calendar, the 1643rd year of the Common Era (CE) and Anno Domini (AD) designations, the 643rd year of the 2nd millennium, the 43rd year of the 17th century, and the 4th year of the 1640s decade. As of the start of 1643, the Gregorian calendar was 10 days ahead of the Julian calendar, which remained in localized use until 1923.

• January 21 – Abel Tasman discovers Tonga
• February 6 – Abel Tasman discovers the Fiji islands.
• May 19 – Battle of Rocroi: French victory over the Spanish at Rocroi, France.
• July 13 – English Civil War: Battle of Roundway Down – In England, Lord Henry Wilmot, Earl of Rochester, commanding the Royalist forces, wins a crushing victory over the Parliamentarian Sir William Waller.
• An Calbhach mac Aodha O Conchobhair Donn inaugurated as the last king of Connacht
• Austro-Bavarian army defeats French at Duttlingen.
• The Royalist forces supporting Charles I in the English Civil War win the Battle of Adwalton Moor and gain control of Yorkshire.
• End of the reign of Empress Meisho of Japan
• Emperor Go-Komyo ascends to the throne of Japan
• Evangelista Torricelli invents the mercury barometer
• New England Confederation is formed
• Battle of Tüttlingen fought
• Baden-Baden pillaged by the French
• John Colgan published the first two volumes of the Acta Sanctorum
• Paul Chomedey de Maisonneuve places the first cross atop Mount Royal
• Åmål is granted its city charter
• Miyamoto Musashi dictates The Book of Five Rings (Go Rin No Sho) to his student, completing it in 1654 just before his death
• Hong Taiji, Emperor of the Qing Dynasty of the Manchu dies and is succeeded by his five-year-old son, the later Shunzhi Emperor of China.
• Roger Williams, a co-founder of Rhode Island, publishes A Key Into the Language of America
• Evangelista Torricelli invented the Barometer.

• January 4 – Sir Isaac Newton, English scientist (d. 1727)