The Railway Mail Service of the United States Post Office Department was a significant mail transportation service in the US from the mid-19th century until the mid-20th century. The RMS, or its successor the Postal Transportation Service (PTS), carried the vast majority of letters and packages mailed in the United States from the 1890s until the 1960s.

The United States Post Office Department was the predecessor of the United States Postal Service, established in 1792. From 1872 to 1971, it was officially in the form of a Cabinet department. It was headed by the postmaster general.