A cofferdam is an enclosure built within a body of water to allow the enclosed area to be pumped out or drained. This pumping creates a dry working environment so that the work can be carried out safely. Cofferdams are commonly used for construction or repair of permanent dams, oil platforms, bridge piers, etc., built within water.

These cofferdams are usually welded steel structures, with components consisting of sheet piles, wales, and cross braces. Such structures are usually dismantled after the construction work is completed.

The origin of the word comes from coffer (originally from Latin cophinus meaning 'basket') and dam from Proto-Germanic *dammaz meaning 'barrier across a stream of water to obstruct its flow and raise its level').

The term is also used in naval architecture, to refer to a space between two watertight bulkheads or decks within a ship.

For dam construction, two cofferdams are usually built, one upstream and one downstream of the proposed dam, after an alternative diversion tunnel or channel has been provided for the river flow to bypass the foundation area of the dam. These cofferdams are typically a conventional embankment dam of both earth- and rock-fill, but concrete or some sheet piling also may be used. Usually, upon completion of the dam and associated structures, the downstream coffer is removed and the upstream coffer is flooded as the diversion is closed and the reservoir begins to fill. Depending on the geography of a dam site, in some applications, a U-shaped cofferdam is used in the construction of one half of a dam. When complete, the cofferdam is removed and a similar one is created on the opposite side of the river for the construction of the dam's other half.

Cofferdams are used in ship husbandry to allow dry access to underwater equipment and to close underwater openings while work is done on the fittings inside the ship. This is more common in naval vessels where a cofferdam may fit several vessels of a class.

The cofferdam is also used on occasion in the shipbuilding and ship repair industry, when it is not practical to put a ship in drydock for repair work or modernization. An example of such an application is the lengthening of ships. In some cases a ship is actually cut in two while still in the water, and a new section of ship is floated in to lengthen the ship. The cutting of the hull is done inside a cofferdam attached directly to the hull of the ship; the cofferdam is then detached before the hull sections are floated apart. The cofferdam is later replaced while the hull sections are welded together again. As expensive as this may be to accomplish, the use of a drydock might be even more expensive.

Cofferdams are also used in some marine salvage operations. Cofferdams have been used to recover aircraft from water as well, as in the case of Avro Lancaster ED603, which was recovered from the IJsselmeer in 2023 using a cofferdam, allowing for close examination of the wreckage, as well as to locate and repatriate the remains of its crew.