Events of 1621

January–June

• February 9 – Pope Gregory XV succeeds Pope Paul V as the 234th pope.

• February 17 – Miles Standish is appointed as the first commander of Plymouth Colony.

• March 16 – Samoset, a Mohegan, visits the settlers of Plymouth Colony and greets them, "Welcome, Englishmen! My name is Samoset."

• March 22 – The Pilgrims of Plymouth Colony sign a peace treaty with Massasoit of the Wampanoags.

• April 1 – The Plymouth, Massachusetts colonists create the first treaty with native Americans.

• April 5 – The Mayflower sets sail from Plymouth on a return trip to England.

• May 24 – The Protestant Union is formally dissolved.

• June 21 – Thirty Years' War: Twenty-seven Czech lords are executed on the Old Town Square in Prague as a consequence of the Battle of White Mountain.

July–December

• October – The Pilgrims of Plymouth Colony and Wampanoags celebrate a harvest feast (3 days), later regarded as the "First Thanksgiving", noted for peaceful co-existence.

• November 11 – The ship Fortune arrives at Plymouth Colony, with 35 more settlers.

Undated

• The Venezuelan city of Petare is founded by Spanish conquistadors as San Jose de Guanarito.

• The Swedish city of Gothenburg is founded by Gustavus Adolphus.

• Riga falls under the rule of Sweden.

• The Swedish king Gustav II Adolf grants city rights to Luleå, Piteå and Torneå (Tornio).

• The Dutch mathematician and astronomer, Willebrord Snel van Royen – Snellius (1580–1626), discovers the famous law of refraction, also known as "Snellius' law".

Births

• January 27 – Thomas Willis, English physician who made important contributions to anatomy, particularly of the brain and nervous system (d. 1675)

• February 2 – Johannes Schefferus, Alsatian-born humanist (d. 1679)

• February 21 – Rebecca Nurse, accused witch (d. 1692)

• March 31 – Andrew Marvell, English poet (d. 1678)

• April 1 – Guru Teg Bahadur, 9th Sikh Guru (d. 1675)

• April 25 – Roger Boyle, 1st Earl of Orrery, British soldier, statesman, and dramatist (d. 1679)

• July 6 – Jean de La Fontaine, French writer (d. 1695)

• July 22 – Anthony Ashley-Cooper, 1st Earl of Shaftesbury, British politician (d. 1683)

• August 19 – Gerbrand van den Eeckhout, Dutch painter (d. 1674)

• September 8 – Louis II de Bourbon, Prince de Condé, French general (d. 1686)

• December 23 – Edmund Berry Godfrey, English magistrate (d. 1678)

• December 23 – Heneage Finch, 1st Earl of Nottingham, Lord Chancellor of England (d. 1682)

Deaths

• January 28 – Pope Paul V (b. 1550)

• February 15 – Michael Praetorius, German composer (b. 1571)

• February 28 – Cosimo II de' Medici, Grand Duke of Tuscany (b. 1590)

• March 31 – King Philip III of Castile (Philip II of Portugal) (b. 1578)

• April 1 – Cristofano Allori, Italian painter (b. 1577)

• April 15 – John Carver, first governor of Plymouth Colony

• May 15 – Hendrick de Keyser, Dutch architect and sculptor (b. 1565)

• June 8 – Anne de Xainctonge, French saint (b. 1567)

• June 21 – Kryštof Harant z Polžic a Bezdružic, Bohemian composer, soldier, and author (executed) (b. 1564)

• July 2 – Thomas Harriot, English astronomer and mathematician (b. c. 1560)

• July 10 – Charles Bonaventure de Longueval, Count of Bucquoy, soldier in Habsburg service (b. 1571)

• July 13 – Archduke Albert of Austria, Governor of the Low Countries (b. 1559)

• August 3 – Guillaume du Vair, French writer (b. 1556)

• August 15 – John Barclay, Scottish writer (b. 1582)

• September 17 – Robert Bellarmine, Italian saint (b. 1542)

• September 24 – Jan Karol Chodkiewicz, Polish military commander (b. 1560)

• September 25 – Mary Sidney, English writer, patroness and translator (b. 1561)

• October 8 – Antoine de Montchrestien, French dramatist and economist (b. c. 1575)

• October 16 – Jan Pieterszoon Sweelinck, Dutch composer (b. 1562)

• November 26 – Ralph Agas, English surveyor (b. c. 1540)

• December 13 – Katarina Stenbock, queen of Gustav I of Sweden (b. 1535)

• December 15 – Charles de Luynes, Constable of France (b. 1578)