A trench coat is a variety of coat made of waterproof heavy-duty fabric, originally developed for British Army officers before the First World War, and becoming popular while used in the trenches, hence the name trench coat.

Originally made from gabardine, a worsted wool fabric waterproofed using lanolin before weaving, the traditional colour of a trench coat was khaki.

Traditionally trench coats are double-breasted with 10 front buttons, wide lapels, a storm flap, and pockets that button-close. The coat is belted at the waist with a self-belt, with raglan sleeves ending in cuff straps around the wrists that also buckle, to keep water from running down the forearm when using binoculars in the rain. The coat often has epaulettes that button-close, which were functional in a military context.

The trench coat was typically worn as a windbreaker or as a rain jacket, and not for sole protection from the cold in winter. Although some may feature a removable wool liner for additional warmth, they are usually not as warm as an overcoat. Period advertisements from the First World War reveal that the trench coat was sized to wear over the British Warm, to offer water protection when the temperature was cold enough to require the heavier coat, which explains the traditionally generous sizing of trench coats. Makers in recent years have resized trench coats downwards to conform more closely to overcoat sizing, as two coats would rarely be worn together today.

Popularised by film stars such as Humphrey Bogart, and Peter Sellers in the Pink Panther films, the trench coat has become a fashion staple, available in many colours, through brands such as Burberry.

The trench coat was developed as an alternative to the heavy serge greatcoats worn by British and French soldiers in the First World War. Invention of the trench coat is claimed by two British luxury clothing manufacturers, Burberry and Aquascutum, with Aquascutum's claim dating back to the 1850s. Thomas Burberry had invented gabardine fabric in 1879 and submitted a design for a British Army officer's raincoat to the War Office in 1901.

The trench coat became an optional item of dress in the British Army, and was obtained by private purchase by officers and Warrant Officers Class I who were under no obligation to own them. No other ranks were permitted to wear them. Another optional item was the British Warm, a wool coat similar to the greatcoat that was shorter in length, also worn by British officers and Warrant Officers Class I as an optional piece.

During the First World War, the design of the trench coat was modified to include epaulettes and D-rings. The shoulder straps were for the attachment of epaulettes or other rank insignia; the D-ring was originally used for attaching map cases, swords, or other equipment to the belt, and there is a popular myth that it was for the attachment of hand grenades.