| 31st Regiment Illinois Volunteer Infantry | |
|---|---|
 Illinois state flag | |
| Active | September 18, 1861, to July 24, 1865 |
| Country | United States |
| Allegiance | Union |
| Branch | Infantry, |
| Engagements | Battle of Belmont Battle of Fort Donelson Battle of Champion Hill Battle of Big Black River Siege of Vicksburg Battle of Kennesaw Mountain Battle of Atlanta Battle of Jonesboro March to the Sea |
The 31st Regiment Illinois Volunteer Infantry, nicknamed the "Dirty-First," was an infantry regiment that served in the Union Army during the American Civil War.