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).

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.

¹ Note, however, that the name "mess kit" may also refer to a compact kit of cooking and eating utensils for use by soldiers and campers, also known as mess tins and mess gear.