1900

* January 2 – U.S. Secretary of State John Hay announces the Open Door Policy, to promote American trade with China.
* January 5 – Henry A. Rowland of Johns Hopkins University announces a theory about the cause of the Earth's magnetism.
* January 6 – Second Boer War: Boers attempt to end the Siege of Ladysmith, which leads to the Battle of Platrand.
* January 9
	+ Italian club SS Lazio is founded in Rome.
	+ The first through passenger train goes from Cairo to Khartoum.
* January 14
	+ Puccini's opera *Tosca* premieres in Rome, Italy.
	+ The U.S. Senate accepts the British-German Treaty of 1899, in which the United Kingdom renounces its claims to the American Samoa portion of the Samoan Islands.

 File:Boers at Spion Kop, 1900 - Project Gutenberg eText 16462.jpg
* January 24 – Second Boer War – Battle of Spion Kop: Boer troops defeat the British Army.

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* January 27 – Boxer Rebellion: Foreign diplomats in Peking, Qing Dynasty China, demand that the Boxer rebels be disciplined.
* January 31 – Datu Muhammad Salleh, leader of the Mat Salleh Rebellion in North Borneo, is shot dead in Tambunan.

### February

Main article: February 1900
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* February 5 – The United Kingdom and the United States sign a treaty for the building of a Central American shipping canal across Central America in Nicaragua.
* February 6 – The International Arbitration Court at The Hague is created, when the Netherlands' Senate ratifies an 1899 peace conference decree.
* February 8 – Second Boer War: British troops defeat the Boers at Ladysmith, South Africa.
* February 14 – Second Boer War: Battle of Paardeberg – 20,000 British troops invade the Orange Free State.
* February 15 – Second Boer War: The Siege of Kimberley is lifted.
* February 16 – The Southern Cross expedition led by Carsten Borchgrevink achieved a new Farthest South of 78° 50'S, making the first landing at the Great Ice Barrier.
* February 17 – Second Boer War: Battle of Paardeberg – British troops defeat the Boers.
* February 27
	+ The British Labour Party is officially established, at a meeting in the Congregational Memorial Hall in London, and Ramsay MacDonald is appointed as its first secretary.
	+ Second Boer War: British military leaders accept the unconditional notice of surrender from Boer General Piet Cronjé.
	+ FC Bayern, Germany's most successful football club, is founded in Munich.

### March

Main article: March 1900
* March 5 – Two U.S. Navy cruisers are sent to Central America to protect American interests in a dispute between Nicaragua and Costa Rica.
* March 6 – A coal mine explosion in West Virginia, United States, kills 50 miners.
* March 14
	+ Botanist Hugo de Vries rediscovers Mendel's Laws of Heredity.
	+ The Gold Standard Act is ratified, placing the United States currency on the gold standard.
* March 16 – British archaeologist Sir Arthur Evans purchases the land on Crete on which the ruins of the Palace of Knossos stand. He begins to unearth some of the palace three days later.
* March 18 – AFC Ajax, a successful football club in Netherlands, is founded in Amsterdam.
* March 23 – Karl Landsteiner first reports his discovery of an accurate means for classifying a system of blood type, which will universally be referred to as the ABO blood group system and for which he will be awarded the Nobel Prize in Physiology or Medicine in 1930.
* March 27 – The arrival of a Russian naval fleet in Korea causes concern to the Imperial Japanese government.

### April

Main article: April 1900
 File:Vue panoramique de l'exposition universelle de 1900.jpg
* April 4 – Queen Victoria arrives in Dublin on a rare visit.
* April 14 – The Exposition Universelle, a world's fair, opens in Paris.
* April 22 – Battle of Kousséri: French forces secure their domination of Chad. Warlord Rabih az-Zubayr is defeated and killed.

### May

Main article: May 1900
* May – American explorer Robert Peary is the first person to sight Kaffeklubben Island.
* May 1 – Scofield Mine disaster: An explosion of blasting powder in a coal mine in Scofield, Utah, United States, kills 200 people.
* May 14 – The second Olympic Games, Paris 1900, open (as part of the Paris World Exhibition).
* May 17
	+ Second Boer War: The British Army relieves the Siege of Mafeking.
	+ Boxer Rebellion: Boxers destroy three villages near Peking, and kill 60 Chinese Christians.
	+ L. Frank Baum's *The Wonderful Wizard of Oz* is published in Chicago, the first of Baum's Oz books, chronicling the fictional Land of Oz for children.
* May 18 – The United Kingdom proclaims a protectorate over Tonga.
* May 21 – Russia invades Manchuria.
* May 24 – Second Boer War: The British annex the Orange Free State, as the Orange River Colony.
* May 28 – Boxer Rebellion: The Boxers attack Belgians, in the Fengtai railway station.
* May 29 – N'Djamena, the capital city of Chad, is founded as *Fort-Lamy*, by French commander Émile Gentil.
* May 31 – Boxer Rebellion: Peacekeepers from various European countries arrive in China, where they join with Japanese forces.

### June

Main article: June 1900
* June 1 – American temperance agitator Carrie Nation begins her crusade to demolish saloons.
* June 5 – Second Boer War: British soldiers take Pretoria.
* June 14 – The Reichstag approves the second of the German Naval Laws allowing expansion of the Imperial German Navy.
* June 17 – Boxer Rebellion – Battle of Dagu Forts: Naval forces of the Eight-Nation Alliance capture the Taku Forts, on the Hai River estuary in China.
* June 20 – Boxer Rebellion: Boxers gather about 20,000 people near Peking, and kill hundreds of European citizens, including the German ambassador.
* June 25 – The Taoist monk Wang Yuanlu discovers the Dunhuang manuscripts, a cache of ancient texts that are of great historical and religious significance, in the Mogao Caves of Dunhuang, China, where they have been sealed since the early 11th century.
* June 30 – Hoboken Docks fire: A wharf fire at the docks in Hoboken, New Jersey, owned by the North German Lloyd Steamship line, spreads to German passenger ships *Template:SS, Template:SS,* and *Template:SS.* The fire engulfs the adjacent piers and nearby ships, killing 326 people.

### July

Main article: July 1900
 File:First Zeppelin ascent.jpg
 File:Map of Australia.png
* July 2 – The first zeppelin airship flight, by Ferdinand Zeppelin's Zeppelin *LZ 1* is carried out over Lake Constance, near Friedrichshafen, Germany.
* July 12 – The new German cruise liner, the SS *Deutschland*, breaks the record for the Blue Riband for the first time with an average speed of Template:Convert.
* July 23–25 – The First Pan-African Conference is held in London.
* July 29 – King Umberto I of Italy is assassinated by Italian-born anarchist Gaetano Bresci in Monza.

### August

Main article: August 1900
* August – The first *Michelin Guide* is published in France.
* August 14 – Boxer Rebellion: An international contingent of troops, under British command, invades Peking and frees the European hostages.
* August 27 – The 1900 Galveston Hurricane forms, going on to kill 8,000 people, the deadliest hurricane in U.S. history, and dissipates on September 15.

### September

Main article: September 1900
* September 8 – The 1900 Galveston hurricane kills about 6,000–12,000 people.
* September 12 – Admiral Fredrik von Otter becomes Prime Minister of Sweden.
* September 13 – Philippine–American War – Battle of Pulang Lupa: Filipino resistance fighters defeat a detachment of American soldiers.
* September 17 – Philippine–American War – Battle of Mabitac: Filipinos under Juan Cailles defeat the Americans, under Colonel Benjamin F. Cheatham.

### October

Main article: October 1900
* October 9 – The Cook Islands become a territory of the United Kingdom.
* October 19 – Max Planck discovers the law of black-body radiation (Planck's law), by introducing the notion of light quanta, leading in 1905 to Albert Einstein's explanation of the photoelectric effect and beginning the Quantum Revolution.
* October 25 – The United Kingdom annexes the Transvaal.

### November

Main article: November 1900
* November 6 – 1900 United States presidential election: Republican incumbent William McKinley is reelected by defeating Democratic challenger William Jennings Bryan in a rematch.
* November 29 – Herbert Kitchener succeeds Frederick Roberts as commander-in-chief of the British forces in South Africa and implements a scorched earth strategy.

### December

Main article: December 1900
* December 14 – Max Planck announces his discovery of the law of black body emission, marking the birth of quantum physics.
* December 19 – Hopetoun Blunder: The first Governor-General of Australia John Hope, 7th Earl of Hopetoun, appoints Sir William Lyne premier of the new state of New South Wales, but he is unable to persuade other colonial politicians to join his government, and is forced to resign.
* December 27 – British human rights activist Emily Hobhouse arrives in Cape Town, South Africa.

### Date unknown

* Australasian prospector Albert Fuller Ellis identifies phosphate deposits on the Pacific Islands of Nauru and Banaba Island (Ocean Island).
* In New Haven, Connecticut, U.S., Louis Lassen of Louis' Lunch makes the first modern-day hamburger sandwich.
* Four out of every 1,000 residents of British India die of cholera each year.

### January

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 File:William Haines 1928 cropped.jpg
 File:Kraljica marija.jpg
 File:Hyman Rickover 1955.jpg
* January 1
	+ Paola Borboni, Italian film actress (d. 1995)
	+ Xavier Cugat, Cuban bandleader (d. 1990)
	+ Hub van Doorne, Dutch businessman (d. 1979)
	+ Chiune Sugihara, Japanese diplomat, saved Jewish WWII refugees (d. 1986)
* January 2
	+ William Haines, American actor (d. 1973)
	+ Józef Klotz, Polish footballer (d. 1941)
* January 3 - Maurice Jaubert, French composer and soldier (d. 1940)
* January 4 - James Bond, American ornithologist (d. 1989)

1. ↑ Template:Cite book
2. ↑ Template:Cite web
3. ↑ *Facts About Association Football - History Timeline* Template:ISBN p. 13
4. ↑ Landsteiner, Karl (1900). "Zur Kenntnis der antifermentativen, lytischen und agglutinierenden Wirkungen des Blutserums und der Lymphe"]. Zbl f. Bakt. 27:357-362 (issue of March 23, 1900)
5. ↑ Template:Cite book
6. ↑ Template:Cite book
7. ↑ "What's Observed in a Rating? Rankings as Orientation in the Face of Uncertainty", by Elena Esposito and David Stark, in *The Performance Complex: Competition and Competitions in Social Life*, ed. by David Stark (Oxford University Press, 2020) p.124
8. ↑ "On this Day: Historic Publication" Template:Webarchive, OnThisDay.com
9. ↑ Template:Cite book
10. ↑ Template:Cite book
11. ↑ Template:Cite book