- May 2 General Motors acquires the Chevrolet Motor Company of Delaware
- May 10 HMS Vindictive sunk to block entrance of Ostend Harbor
- May 11 44th Kentucky Derby: William Knapp on Exterminator wins in 2:10.8
- May 13 1st US airmail stamps issued (24 cents)
- May 14 Sunday baseball is made legal in Washington, D.C.
- May 15 1st regular US airmail postal service between NY, Philadelphia & Washington, D.C.
- May 15 43rd Preakness: Johnny Loftus aboard War Cloud wins in 1:53.6
- May 15 Finnish Civil War ends
- May 15 Greeks troops land at Smyrna
- May 15 Washington Senator Walter Johnson pitches 1-0, 18 inning game
- May 16 The Sedition Act of 1918 is passed by the U.S. Congress, making criticism of the government an imprisonable offense.
- May 18 Dutch Indian Volksraad installed in Batavia
- May 18 TNT explosion in chemical factory in Oakdale, Pennsylvania, kills 200 people
- May 19 Washington's 1st Sunday game, Senators beat Cleveland 1-0 in 18 innings
- May 20 US Navy launches 'USS New Mexico', its 1st battleship with electric-turbine propulsion system
- May 21 US House of Representatives passes amendment allowing women to vote
- May 23 King Oil/Shell refinery on Curacao officially opens
- May 24 British officer General Poole lands at Murmansk, the Russian port on the Barents Sea
- May 24 Cleveland starter Stan Coveleski sets club record for most innings pitched (19) in a complete game as the Indians beat the New York Yankees, 3-2 at the Polo Grounds, NYC
- May 26 Armenia defeats the Ottoman Army in the Battle of Sardarapat
- May 26 Georgian Social Democratic Republic declares independence from Russia
- May 27 Third Battle of Aisne: German offensive overcomes British forces (WWI)
- May 28 Azerbaijan gains independence and declares itself a Democratic Republic
- May 28 The Armenian National Council declares Armenia independent from the Russian Empire
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