The 90th Illinois Infantry Regiment (Second Irish) was an Illinois regiment of the Union Army that was active from 7 September 1862 to 10 June 1865 during the American Civil War. Recruited at Rockford, Galva, Lockport, LaSalle, Joliet, Chicago, Belvidere, and Ottawa, the regiment fought at the Siege of Vicksburg, in Tennessee, in the Atlanta campaign, the March to the Sea, and the Carolinas campaign.