Barbed wire, also known as barb wire, is a type of steel fencing wire constructed with sharp edges or points arranged at intervals along the strands. Its primary use is the construction of inexpensive fences, and it is also used as a security measure atop walls surrounding property. As a wire obstacle, it is a major feature of the fortifications in trench warfare.

A person or animal trying to pass through or over barbed wire will suffer discomfort and possibly injury. Barbed wire fencing requires only fence posts, wire, and fixing devices such as staples. It is simple to construct and quick to erect, even by an unskilled person.

The first patent in the United States for barbed wire was issued in 1867 to Lucien B. Smith of Kent, Ohio, who is regarded as the inventor. Joseph F. Glidden of DeKalb, Illinois, received a patent for the modern invention in 1874 after he made his own modifications to previous versions.

Wire fences are cheaper and easier to erect than their alternatives (one such alternative is Osage orange, a thorny bush that is time-consuming to transplant and grow). When wire fences became widely available in the United States in the late 19th century, it became more affordable to fence much larger areas than before, and intensive animal husbandry was made practical on a much larger scale.

An example of the costs of fencing with lumber immediately prior to the invention of barbed wire can be found with the first farmers in the Fresno, California, area, who spent nearly $4,000 (equivalent to $90,000 in 2021) to have wood for fencing delivered and erected to protect 2,500 acres (1,000 ha) of wheat crop from free-ranging livestock in 1872.