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1839 (MDCCCXXXIX) was a common year starting on Tuesday of the Gregorian calendar and a common year starting on Sunday of the Julian calendar, the 1839th year of the Common Era (CE) and Anno Domini (AD) designations, the 839th year of the 2nd millennium, the 39th year of the 19th century, and the 10th and last year of the 1830s decade. As of the start of 1839, the Gregorian calendar was 12 days ahead of the Julian calendar, which remained in localized use until 1923.
- April 9 – The world's first commercial electric telegraph line comes into operation, alongside the Great Western Railway line in England, from London Paddington station to West Drayton.
- April 19 – The Treaty of London establishes Belgium as a kingdom, with its independence and neutrality guaranteed by the great powers of Europe. Half of the Limburg province of Belgium is added to the Netherlands, giving rise to a Belgian Limburg and Dutch Limburg (the latter being joined (from September 5) to the German Confederation).
- April 24 – Boston University is established as the Newbury Biblical Institute in Vermont.
- May 7 – The Bedchamber Crisis begins in the United Kingdom, after Prime Minister Lord Melbourne announces his resignation.  Queen Victoria asks several MPs to form a new government, and they insist on the condition that the Queen dismiss several of her personal attendants, the ladies of the bedchamber, for political reasons.
- May 12 – Socialist activist Louis Auguste Blanqui and the Société des Saisons begin an uprising against the government of France. The insurrection is suppressed, but not before 50 people are killed and 190 wounded. Blanqui is imprisoned until 1848. 
- May 22 – Former British statesman Lord Durham, as President of the New Zealand Company, formally asks the British government for permission to colonize New Zealand, and to establish a colonial government under the sovereignty of the United Kingdom. 
- May 23 – Turkish troops cross the Euphrates River and invade Syria, but are defeated in battle in June. 
- June 3 – Destruction of opium at Humen begins, casus belli for Britain to open the 3-year First Opium War against Qing Dynasty China. A rapid rise in the sale of opium in China to over 40,000 chests (~56,000 kilograms (123,000 lb)) per annum results.  has caused the Chinese government to dispatch scholar-official Lin Zexu to Guangzhou to deal with the growing problem of opium addiction.
- June 22 – Louis Daguerre receives a patent for his camera (commercially available by September at the price of 400 francs).
- October 3 – A railway between Naples and Portici (7.4 km (4.6 mi)) in the Kingdom of the Two Sicilies is inaugurated by King Ferdinand II of Bourbon as the first line in the Italian Peninsula.
- October 15 – Emir Abdelkader declares a jihad against the French.
- November 4 – Newport Rising: Between 5,000 and 10,000 Chartist sympathisers march on Newport, Monmouthshire, to liberate Chartist prisoners; around 22 are killed when troops fire on the crowd. This is the last large-scale armed civil rebellion against authority in mainland Britain and sees the most deaths.
- November 11 – The Virginia Military Institute is founded in Lexington, Virginia.
- November 17 – Giuseppe Verdi's first opera, Oberto, conte di San Bonifacio, opens in Milan.
- November 25 – A disastrous cyclone hits India with terrible winds and a giant 40-foot storm surge, wiping out the port city of Coringa; 300,000 people die.
- November 27 – The American Statistical Association is founded in Boston, Massachusetts.
- December 6 – The Whig Party (United States), at its first ever national convention, in Harrisburg, Pennsylvania, nominates former U.S. Army General William Henry Harrison to be its candidate for President of the United States in the 1840 election. Although Senator Henry Clay of Kentucky has received 103 of the 128 necessary votes on the first ballot, he obtains only 90 on the final vote, while Harrison gets 148. Former U.S. Senator John Tyler is unanimously nominated for Vice President.
- In the United States, the first state law permitting women to own property is passed in Jackson, Mississippi.
- The United Kingdom of Great Britain and Ireland, backed by the Russian Empire and the Austrian Empire, compels July Monarchy France to abandon Muhammad Ali of Egypt, and forces him to return Syria and Arabia to the Ottoman Empire.
- Tanzimat starts in the Ottoman Empire.
- Emperor Minh Mạng renames Việt Nam to Đại Nam.
- "Experimental Researches in Electricity", clarifying the true nature of electricity.
- Charles Goodyear vulcanizes rubber.
- An archaeological excavation on Copán begins.
- Khalid bin Saud Al Suad usurps the throne from Faisal bin Turki bin Abdullah Al Saud, who assumed power of Nejd in 1834, and is sent to Cairo as prisoner. Omar bin Ofaysan, the Amir Faisal’s governor in the Eastern Province seeks asylum in Bahrain, but Khalid the pretender demands his surrender and the surrender of the fort at Dammam; then under the control of the Al Khalifa of Bahrain.
- Khorshid Pasha vows to attack Bahrain to exert Egyptian rule over Bahrain, but his attack is prevented after Shaikh Abdulla bin Ahmed of Bahrain pays tribute
- A quarrel broke out between the Chief of Abu Dhabi of the Beniyas tribe, Shaikh Khalifa bin Shakboot, and the fugitives who settled there after their departure from Bahrain, the Al Binali tribe. Under the command of their leader, Isa bin Tureef Al Binali, they relocate to Kenn Island where they exercise depredations over the Bahrain and other Gulf vessels. Their motive, to restore their belongings which they abandoned upon leaving Bahrain.