1905 (MCMV) was a common year starting on Sunday of the Gregorian calendar and a common year starting on Saturday of the Julian calendar, the 1905th year of the Common Era (CE) and Anno Domini (AD) designations, the 905th year of the 2nd millennium, the 5th year of the 20th century, and the 6th year of the 1900s decade. As of the start of 1905, the Gregorian calendar was 13 days ahead of the Julian calendar, which remained in localized use until 1923.
As the second year of the massive Russo-Japanese War begins, more than 100,000 die in the largest world battles of that era, and the war chaos leads to the 1905 Russian Revolution against Nicholas II of Russia (Shostakovich's 11th Symphony is subtitled The Year 1905 to commemorate this) and the start of Revolution in the Kingdom of Poland. Canada and the U.S. expand west, with the Alberta and Saskatchewan provinces and the founding of Las Vegas. 1905 is also the year in which Albert Einstein, at this time resident in Bern, publishes his four Annus Mirabilis papers in Annalen der Physik (Leipzig) (March 18, May 11, June 30 and September 27), laying the foundations for more than a century's study of theoretical physics.
January 1 – In a major defeat in the Russo-Japanese War, Russian General Anatoly Stessel surrenders Port Arthur, located on mainland China, to the Japanese.
January 3 – Japan formally repossesses Port Arthur, and renames it Ryojun, holding it for the next 40 years. The area will revert in 1945 to China, and become the Lushunkou District.
Gheorghe Grigore Cantacuzino becomes Prime Minister of Romania for the second time, having previously served from 1899 to 1900, and remains in office for more than two years.
The city of Bend, Oregon, plotted out in 1900 by Alexander Drake, is incorporated as a town for local logging companies, and will have a population of 536 in 1910. By the year 2020, it will have almost 100,000 residents.
January 5 – Baroness Emma Orczy's play The Scarlet Pimpernel, the forerunner of her novel, opens at the New Theatre in London, beginning a run of 122 performances and numerous revivals.
January 6 – The Lick Observatory announces the discovery on 3 December 1904 of a sixth moon of Jupiter, made by their astronomer Charles D. Perrine.
January 11 – Under the supervision of five editors, work begins on the comprehensive Catholic Encyclopedia, subtitled "An International Work of Reference on the Constitution, Doctrine, Discipline, and History of the Catholic Church." The first volume will appear in 1907.
January 15 – A series of three 41 metres (135 ft) high tsunamis kill 61 people in Norway, after a rockslide sweeps down Mount Ramnefjell and crashes into Lake Lovatnet.
January 17 – In France, Prime Minister Émile Combes and his cabinet announce their resignations after being implicated in the Affair of the Cards (L'Affaire des Fiches), a system set up by the War Ministry to purge the French Army officers corps of Jesuits.
January 21 – The Dominican Republic signs an agreement with the United States to allow the U.S. to administer the collection of customs taxes for Santo Domingo for 50 years, with the U.S. to assume responsibility for payment of the Republic's debts to foreign nations from Dominican income. The agreement is done as an exercise of the "Roosevelt Corollary" to the Monroe Doctrine.
January 22 (January 9 O.S.) – The Bloody Sunday massacre of peaceful Russian demonstrators at the Winter Palace in Saint Petersburg takes place, leading to an unsuccessful uprising.
January 26 – (January 13 O.S. in Russia)
Russian Revolution of 1905: The Imperial Russian Army opens fire on demonstrators in Riga, Governorate of Livonia, killing 73 people and injuring 200.
Elections are held in Hungary for the Országgyűlés, the Kingdom's parliament within Austria-Hungary. Voters overwhelmingly reject the Liberal Party, led by Prime Minister István Tisza, that has ruled Hungary since 1875. The Liberals lose 118 of their 277 seats, but Emperor Franz Joseph I of Austria-Hungary ignores the results and keeps Tisza in power.
January 27 – The Nelson Act is passed into law in the United States, providing for racial segregation of schools in the Alaska Territory.
January 29 – Rioting breaks out in Warsaw, at this time under Russian Imperial rule with a Russian Governor-General.