The 123rd Regiment Illinois Volunteer Infantry, was an infantry and mounted infantry regiment that served in the Union Army during the American Civil War. In 1863 and 1864 it was temporarily known as the 123rd Illinois Volunteer Mounted Infantry Regiment, as part of Wilder's Lightning Brigade.

John Thomas Wilder was an officer in the Union Army during the American Civil War, noted principally for capturing the critical mountain pass of Hoover's Gap during the Tullahoma Campaign in Central Tennessee in June 1863. Wilder had personally ensured that his "Lightning Brigade" of mounted infantry was equipped with the new Spencer repeating rifle. However, Wilder initially had to appeal to his men to pay for these weapons themselves before the government agreed to carry the cost. The victory at Hoover's Gap was attributed largely to Wilder's persistence in procuring the new rifles, which disoriented the enemy.