The 1st Maryland Infantry was a Confederate States Army regiment that was active from April 1861 to August 1862 during the American Civil War. It was formed by Maryland secessionist volunteers who chose to fight for the Confederacy, even as their state rejected secession and remained loyal to the Union. The regiment fought at the First Battle of Bull Run, the Valley Campaign, and the Peninsula Campaign, and, after it was mustered out in April 1862, most of its men joined the 2nd Maryland Infantry, CSA rather than return to Union-controlled Maryland.