The Trans-Mississippi was a name applied to a region of the United States in the 19th century. The Trans-Mississippi consisted of the states of Arkansas, Louisiana, Missouri and Texas, and the Indian Territory. This term was especially used by the Confederate States of America, which had its Trans-Mississippi headquarters at Shreveport, Louisiana and Marshall, Texas. The geographical term is used today only in matters relating to the study of the American Civil War.

In 1898, a Trans-Mississippi Exposition was held at Omaha, Nebraska. The exposition itself is long-forgotten, but the postage stamps of the Trans-Mississippi Exposition Issue are considered some of the most beautiful stamps ever issued by the US, and a complete set of the "Trans-Miss" is highly prized. In 1998, a set of stamps using designs derived from the original issue was issued to commemorate its 100th anniversary.