Sandbag may refer to a Nintendo character, Sandbag

A sandbag is typically used in flood control, but the exact use can vary.

Sandbags are burlap bags filled with sand or soil, in an amount that a person can carry from where it is filled, to where it will be used. The use most people are familiar with is water control during emergencies, when rivers are threatening to flood outside their normal channel, or if a levee or dike is damaged. However, sandbags often are used in non-emergency situations (or after the emergency has passed) as the foundation for new (or just higher) levees, or other water-control structures.

In military science, sandbags are also used for field fortifications, or as a temporary protective measure for civilian structures. Because burlap and soil are very inexpensive, quite thick protective structures can be erected cheaply. The friction generated between moving soil or sand grains, and the multiple tiny air gaps, also make sandbags quite efficient dissipators of explosive blast. Sandbags used in fortification generally have dimensions carefully calculated so that the finished bags can be interlocked like brickwork, and are of such a weight as to maximise the efficiency of manual labour in filling and laying them. They may be laid in excavated defences as revetment, or as free-standing walls above ground where excavations are impractical. Plain burlap sandbags deteriorate fairly quickly, so if it is anticipated that a sandbag structure will remain in place for a long time, it may be painted with a portland cement slurry to reduce the effects of rot and abrasion.

Sandbags are also used for disposable ballast in hot air balloons, and as counterweights for theatre sets.

Historically, sandbags have typically been filled manually by people wielding shovels, although machines able to fill bags at a much greater rate have been in use since the 1990s.

Sandbag can also refer to a crude weapon consisting of a small bag filled with sand for use as a cudgel typically by criminals, or to the act of striking a person on the head with such a weapon. This usage is obsolescent in normal speech, appearing mainly in legal codes. However the verb form is extended metaphorically in several slang expressions.

In poker, to "sandbag" is to conceal how good a hand you have by betting only small amounts incrementally and deceiving your opponents into staying in the game until finally revealing the true value of your hand and winning the pot. This expression passed on into golf, where a "sandbagger" is someone who misrepresents his handicap to gain an advantange over other players.