20 January – Donald Trump is sworn in as 45th President of the United States.
21 January – Millions of people worldwide join the Women's March in response to the inauguration of Donald Trump as President.
11 February – North Korea prompts international condemnation by test firing a ballistic missile across the Sea of Japan.
10 March – The UN warns that the world is facing the biggest humanitarian crisis since World War II, with up to 20 million people at risk of starvation and famine in Yemen, Somalia, South Sudan and Nigeria.
29 March – The United Kingdom triggers article 50 of the Lisbon Treaty, starting the Brexit Negotiations, the talks for the United Kingdom to leave the European Union.
30 March – SpaceX conducts the world's first reflight of an orbital class rocket.
6 April – In response to a suspected chemical weapons attack on a rebel-held town, the U.S. military launches 59 Tomahawk cruise missiles at an air base in Syria. Russia describes the strikes as an "aggression", adding they significantly damage U.S.–Russia ties.
13 April – In the 2017 Nangarhar airstrike, the U.S. drops the GBU-43/B MOAB, the world's largest non-nuclear weapon, at an ISIL base in Afghanistan.
12 May – Computers around the world are hit by a large-scale ransomware cyberattack, which goes on to affect at least 150 countries.
22 May – A terrorist bombing at an Ariana Grande concert in Manchester, England, kills 22 people and injures over 500.
1 June – Amidst widespread criticism, the U.S. announces its decision to withdraw from the Paris Climate Agreement.
7 June – Two terrorist attacks are simultaneously carried out against the Iranian Parliament building and the Mausoleum of Ruhollah Khomeini, leaving 17 civilians dead and 43 wounded; the first ISIL attack in Iran.
10 June – The 2017 World Expo is opened in Astana, Kazakhstan.
21 June – The Great Mosque of al-Nuri in Mosul, Iraq, is destroyed by ISIL.
25 June – The World Health Organization estimates that Yemen has over 200,000 cases of cholera.
4 July – Russia and China urge North Korea to halt its missile and nuclear programs after it successfully tested its first intercontinental ballistic missile.
7 July – The Treaty on the Prohibition of Nuclear Weapons is voted for by 122 of the 193 UN member states.
10 July – Iraqi Civil War: Mosul is declared fully liberated from the Islamic State of Iraq and the Levant.
5 August – The UN Security Council unanimously approves fresh sanctions on North Korean trade and investment.
17 August – The first observation of a collision of two neutron stars (GW170817).
21 August – A total solar eclipse (nicknamed "The Great American Eclipse") is visible within a band across the entire contiguous United States of America, passing from the Pacific to the Atlantic coasts.
25 August – A military operation targeting Rohingya Muslims in Myanmar "seems a textbook example of ethnic cleansing", according to the United Nations High Commissioner for Human Rights.
25-30 August – Hurricane Harvey strikes the United States as a Category 4 hurricane, causing catastrophic damage to the Houston metropolitan area, mostly due to record-breaking floods. At least 90 deaths are recorded, and total damage reaches $198.6 billion (2017 USD), making Harvey the costliest natural disaster in United States history.
1 September – Russian President Vladimir Putin expels 755 diplomats in response to United States sanctions.
3 September – North Korea conducts its sixth and most powerful nuclear test.
6-10 September – The Caribbean and United States are struck by Hurricane Irma, a Category 5 hurricane that is the strongest hurricane ever recorded in the Atlantic basin outside the Caribbean and Gulf of Mexico. The storm causes at least 134 deaths and at least $63 billion (2017 USD) in damage.
13 September – The International Olympic Committee awards Paris and Los Angeles the right to host the 2024 and 2028 Summer Olympics, respectively.
15 September – Cassini–Huygens ends its 13-year mission by plunging into Saturn, becoming the first spacecraft to enter the planet's atmosphere.
19 September – On the 32nd anniversary of the deadly 1985 Mexico City earthquake, a magnitude 7.1 earthquake strikes central Mexico, killing more than 350, leaving up to 6,000 injured and thousands more homeless.
19-20 September – Just two weeks after Hurricane Irma struck the Caribbean, Hurricane Maria strikes similar areas, making landfall on Dominica as a Category 5 hurricane, and Puerto Rico as a Category 4 hurricane. Maria causes at least 94 deaths and damages estimated in excess of $103 billion (2017 USD).
25 September – Iraqi Kurdistan votes in a referendum to become an independent state, in defiance of Iraq; by October 15, the crisis escalates into a short-lived armed conflict over disputed territories.
1 October – 58 people are killed and 546 injured when Stephen Paddock opens fire on a crowd in Las Vegas, surpassing the 2016 Orlando nightclub shooting as the deadliest mass shooting perpetrated by a lone gunman in U.S. history.
12 October – The United States announces its decision to withdraw from UNESCO, and is immediately followed by Israel.
14 October – A massive blast caused by a truck bombing in Mogadishu, Somalia kills at least 512 people and injures 316 others.
17 October – Syrian Civil War: Raqqa is declared fully liberated from the Islamic State of Iraq and the Levant.
25 October – At the 19th National Congress of the Communist Party of China, Xi Jinping assumes his second term as General Secretary (China's paramount leader).
27 October – Catalonia declares independence from Spain, but the Catalan Republic is not recognised by the Spanish government or any other sovereign nation.
2 November – A new species of orangutan is identified in Indonesia, becoming the third known species of orangutan, as well as the first great ape to be described for almost a century.
3 November – Syrian Civil War: both Deir ez-Zor in Syria and Al-Qa'im in Iraq are declared liberated from ISIL on the same day.
5 November – The German newspaper Süddeutsche Zeitung publishes 13.4 million documents leaked from the offshore law firm Appleby, along with business registries in 19 tax jurisdictions that reveal offshore financial activities on behalf of politicians, celebrities, corporate giants and business leaders. The newspaper shared the documents with the International Consortium of Investigative Journalists and asked it to lead the investigation.
12 November – A magnitude 7.3 earthquake strikes the border region between Iraq and Iran, leaving at least 530 dead and over 70,000 homeless.
15 November – Zimbabwean President Robert Mugabe is placed under house arrest, as the military take control of the country. He resigns six days later, after 37 years of rule.
15 November – A Leonardo da Vinci painting, Salvator Mundi, sells for US$450 million at Christie's in New York, a new record price for any work of art.
20 November – Nature publishes an article recognising the high velocity asteroid ʻOumuamua as originating from outside the Solar System, i.e. the first known interstellar object.
22 November – The International Court of Justice finds Ratko Mladić guilty of genocide committed in Srebrenica during the 1990s Bosnian War, the worst massacre in Europe since World War II. He is sentenced to life in prison.
24 November – A mosque attack in Sinai, Egypt kills 305 worshippers and leaves hundreds more wounded.
5 December – Russia is banned from the 2018 Winter Olympics in Pyeongchang by the International Olympic Committee, following an investigation into state-sponsored doping.
6 December – The United States officially recognises Jerusalem as Israel's capital.
9 December – The Iraqi military announces that it has "fully liberated" all of Iraq's territory from "ISIS terrorist gangs" and retaken full control of the Iraqi-Syrian border.
14 December – The Walt Disney Company announces that it will acquire most of 21st Century Fox, including the 20th Century Fox film studio, for $66 billion.
22 December – The UN Security Council votes 15–0 in favor of additional sanctions on North Korea, including measures to slash the country's petroleum imports by up to 90%.
24 December – Guatemala follows in the footsteps of the United States by announcing that they will also move their Israeli embassy to Jerusalem, followed by Honduras and Panama two days later.