| Department of Pennsylvania | |
|---|---|
 Pennsylvania flag | |
| Active | April 27 - July 25, 1861 |
| Country |  United States of America |
| Branch |  United States Army |
| Type | Field Army |
| Engagements | American Civil War |
| Commanders | |
| Notable commanders | Robert Patterson |
The Department of Pennsylvania (or General Patterson's Army) was a large military unit in the Union Army at the outset of the American Civil War. Established on April 27, 1861, its territory consisted of Pennsylvania, Delaware, and all of Maryland not embraced in the Department of Annapolis (later renamed Department of Maryland) and the Department of Washington. Its remnants were absorbed into the short-lived Department of the Shenandoah on July 19, 1861, which also absorbed the Department of Maryland on July 25, and on August 24 was merged into the Department of the Potomac.