A tonneau (US: /tʌˈnoʊ/ or UK: /ˈtɒnoʊ/) is an area of a car, truck, or boat open at the top. It can be for passengers or cargo. When applied to trucks it refers to their bed (American English) or tray (British English).

A tonneau was originally an open rear passenger compartment, rounded like a barrel, on an automobile and, by extension, a body style incorporating such a compartment. The word is French, meaning 'cask' or barrel, cf. “tun”.

Early tonneaus normally had a rear-facing hinged door, but single and dual side doors were soon^[when?] introduced.

When the street was muddy or dirty, the car could be backed up to the curb so tonneau passengers could exit directly onto the sidewalk.

Historically, tonneau cover referred to a flexible temporary cover (typically made out of leather or canvas) for an open area of a vehicle.

In current automotive terminology it typically refers to a hard or soft cover that spans the back of a pickup truck to protect the load from weather elements or thieves, or to improve aerodynamics. Tonneau covers come in many styles that fold, retract, or tilt open, and can be locked shut. Common materials used include steel, aluminium, canvas, PVC, fibreglass, and carbon fibre.

Tonneau covers are also used to cover and protect open areas of boats. Many of these covers are made of waterproofed canvas and are held in place by snaps.

The older, original tonneau covers were used to protect unoccupied passenger seats in convertibles and roadsters, and the cargo bed of a pickup truck or coupé utility.