Fiberglass or fibreglass is material made from extremely fine fibers of glass. It is widely used in the manufacture of insulation and textiles. It is also used as a reinforcing agent for many plastic products, the result being a composite material called glass-reinforced plastic or GRP.

Glassmakers throughout history had experimented with glass fibers, but innovations such as fiberglass were only made possible with the advent of finer machine-tooling. In 1893, Edward Drummond Libbey exhibited a dress at the World Columbian Exposition incorporating glass fibers with the diameter and texture of silk fibers.

What is commonly known as "fiberglass" today, however, was invented in 1938 by Russell Games Slayter of Owens-Corning as a material to be used as insulation. It is marketed under the trade name Fiberglas.

The term 'fiberglass' is often used, rather imprecisely, for glass-reinforced plastic (GRP) - see genericized trademark