January–June
- January 3 – Jaroslav Hasek, Czech writer (b. 1883)
- January 9
- Katherine Mansfield, British novelist (b. 1888)
- Edith Thompson, British murderer (hanged) (b. 1893)
- January 11 – Constantine I of Greece, King of Greece (b. 1868)
- January 18 – Wallace Reid, American actor (b. 1891)
- January 23 – Max Nordau, Hungarian author, philosopher, and Zionist leader (b. 1849)
- January 31 – Eligiusz Niewiadomski, Polish artist, political activist and assassin (executed) (b. 1869)
- February 1 – Ernst Troeltsch, German theologian (b. 1865).
- February 3 – Kuroki Tamemoto, Japanese military leader (b. 1844)
- February 10 – Wilhelm Röntgen, German physicist, Nobel Prize laureate (b. 1845)
- February 23 – Théophile Delcassé, French statesman (b. 1852)
- March 8 – Johannes Diderik van der Waals, Dutch physicist, Nobel Prize laureate (b. 1837)
- March 26 – Sarah Bernhardt, French actress (b. 1844)
- March 27 – Sir James Dewar, Scottish chemist (b. 1842)
- March 28 – Michel-Joseph Maunoury, French general (b. 1847)
- April 4 – John Venn, British mathematician (b. 1834)
- April 5 – George Herbert, 5th Earl of Carnarvon, English financier of Egyptian excavations (b. 1866)
- April 23 – Princess Louise of Prussia, Grand Duchess of Baden (b. 1838)
- June 9 – Princess Helena of the United Kingdom, third daughter of Queen Victoria (b. 1846)
- June 10 – Pierre Loti, French writer and naval officer (b. 1850)
- June 24 – Edith Södergran, Finnish author (b. 1892)