Mess dress is the military term for the formal evening dress worn in the mess or at other formal occasions. It is also known as mess uniform and mess kit¹. Its form varies according to regiment, corps or service, but generally a short mess jacket is worn, which either fastens at the neck (being cut-away to show the waistcoat; this is the usual style in cavalry regiments), or is worn with a white shirt and black bow tie (the usual style for all other regiments, corps and services). Most UK regiments' mess dress incorporates high-waisted, very tight trousers known as "overalls", the bottoms of which buckle under heeled boots or "mess wellies". Ornamental spurs are usually worn in cavalry regiments; some other regiments and corps prescribe spurs for "field officers" (majors and above) since in former times these officers would have been mounted.

Mess dress is generally worn as the military equivalent of white tie or black tie. However, in the Royal Navy and some other navies distinguish between mess dress (also known as ball dress), which is the equivalent of white tie, and mess undress, which is the equivalent of black tie. Today the only difference between mess dress and mess undress in the Royal Navy is the colour of the waistcoat, which is white for mess dress and blue (or replaced with a cummerbund) for mess undress. However, before 1939 ball dress required a tailcoat and gold epaulettes. Officers of the rank of Captain RN and above still wear tailcoats for both mess dress and mess undress.

Mess dress in the U.S. Armed Forces is a more recent trend, which started in the early 20th Century. In 1902, when the U.S. Army introduced its last standing collar dress blue uniform, an evening dress uniform, a modified civilian "tail coat" was introduced, and was worn with a white tie and vest. After World War II, the evening dress and mess dress uniforms were reintroduced, with the "tail coat" having a single "Austrian knot" over the branch-of-service color (General Officers had stars over an oak-leaf braid), with the rank placed in the bottom opening of the knot, while the mess coat, for black-tie affairs, used an Austrian knot rank system, with the branch insignia at the bottom. The number of knots indicated the officers rank: five for Colonel, four for Lt. Colonel, three for Captain, two for First Lieutenant, and none for Second Lieutenant. This complicated system was replaced with the evening coat style (which lost its "tails" in the late 1960's) in 1972, using a single knot and the rank placed above the branch-of-service color. A white mess coat, for summertime wear, was introduced in the 1950's.