The 18th Ohio Volunteer Infantry (or 18th OVI) was an infantry regiment in the Union Army during the American Civil War.

Companies A, C, and E enrolled at Ironton, Ohio, on April 22, 1861. Company B enrolled at Marietta, Ohio, on April 27, 1861. Company D enrolled at McArthur, Ohio, on April 18, 1861. Company F enrolled at Gallipolis, Ohio, on April 22, 1861. Company I enrolled at Jackson, Ohio, on April 24. And Company K enrolled at Beverly, Ohio, on April 23, 1861.

The 18th Ohio Infantry organized at Parkersburg, Virginia, and mustered in May 29, 1861, under Colonel Timothy Robbins Stanley in response to President Lincoln's call for 75,000 volunteers. Companies were sent to different points on the Baltimore & Ohio Railroad to guard the railroad and trains between Parkersburg and Clarksburg, West Virginia (then Virginia), until August.

The regiment mustered out at Columbus, Ohio, on August 28, 1861.