25th Pennsylvania Infantry Regiment

The 25th Pennsylvania Volunteer Infantry was a Three Months' Service infantry regiment which served with the Union Army during the American Civil War. Composed of Pennsylvania First Defenders, the first five companies of men who responded to President Lincoln's call for 75,000 volunteers to defend the national capital in Washington, D.C. following the Fall of Fort Sumter to Confederate States Army troops in mid-April 1861, and five additional companies of early responders who enlisted later in April 1861.

The 25th regiment's formation was unusual in that its members mustered in at multiple locations and were subsequently divided among various duty stations, including Washington Arsenal, the U.S. Capitol, and Washington Navy Yard in Washington, D.C., and Fort Washington in Maryland before roughly half of the regiment was moved to Rockville, Maryland and assigned to the Union force commanded by Major-General Robert Patterson.