- Sep 11 Mountain Meadows Massacre, Mormons dressed as Indians murder 120 colonists in Utah
- Sep 12 423 die when "Central America" sinks off Cape Romain SC
- Sep 15 Timothy Alder of NY patents a typesetting machine
- Sep 16 Mexican constitution of force (fiercely attacked by Pope Pius IX)
- Sep 23 Russian warship Leffort disappears in a storm in the Gulf of Finland; 826 die
- Sep 25 Relief of Lucknow by Havelock & Outram begins
- Sep 30 US occupies Sand, Baker, Howland & Jarvis Is south of Hawaii