A hair dryer, hairdryer or blow dryer is an electromechanical device that blows ambient or hot air over damp hair to speed the evaporation of water to dry the hair. Blow dryers enable better control over the shape and style of hair, by accelerating and controlling the formation of temporary hydrogen bonds within each strand. These bonds are powerful (allowing stronger hair shaping than the sulfur bonds formed by permanent waving products) but are temporary and extremely vulnerable to humidity. They disappear with a single washing of the hair.

Hairstyles using blow dryers usually have volume and discipline, which can be further improved with styling products, hairbrushes, and combs during drying to add tension, hold and lift.

Blow dryers were invented in the late 19th century. The first, stationary, model was created by Alexander F. Godefroy in his salon in France. The handheld, household hair dryer first appeared in 1920. Blow dryers are used in beauty salons by professional stylists and in the household by consumers.