A wet sump is a lubricating motor oil management design for four-stroke cycle piston internal combustion engines which uses a built-in reservoir for oil, as opposed to an external or secondary reservoir used in a dry sump design.

Four-stroke engines are lubricated by oil which is pumped into various bearings and thereafter allowed to drain to the base of the engine. In most production cars and motorcycles, which use a wet sump system, the oil is collected in a three to seven quart (3 to 7 L) capacity pan at the base of the engine, known as the oil pan where it is pumped back up to the bearings by the oil pump, internal to the engine.

A wet sump offers the advantage of a simple design, using a single pump and no external reservoir. Since the sump is internal, there is no need for hoses or tubes connecting the engine to an external sump which may leak. An internal oil pump is harder to replace, but that is dependent on the engine design.

Early stationary engines employed a small scoop on the extremity of the crankshaft or connecting rod to assist with the lubrication of the cylinder walls by means of a splashing action.