A trench coat or trenchcoat 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.

Originally made from gabardine, a worsted wool fabric waterproofed using lanolin before weaving, the traditional colour of a trench coat was khaki.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