The Post Office Department was the former name of the United States Postal Service when it was a cabinet department. It was headed by the United States Postmaster General.

The Postal Service Act signed by President George Washington on February 20, 1792 established the Department.

During the Civil War, postal services in the Confederacy were provided by the Confederate Post-office Department , headed by Postmaster General John Henninger Reagan.

The Postal Service Reorganization Act signed by President Richard Nixon on August 12, 1970, replaced the cabinet-level Post Office Department with the government-owned corporation, the United States Postal Service. The Act took effect on July 1, 1971.