A mountain bike or mountain bicycle (abbreviated MTB or ATB (all terrain bicycle)) is a bicycle designed for mountain biking, either on dirt trails or other unpaved environments. In contrast, road bicycles are not designed for such rugged terrain.

Mountain bikes have wide, knobby tires for extra traction and shock absorption. In recent years, front wheel suspension has become the norm and full front and rear suspension is becoming increasingly common. Some mountain bikes are also fitted with bar ends on the handlebars, but with the increase in popularity of riser handlebars (as opposed to a flat straight handlebar) fewer riders use bar end extensions. The bikes normally have 26 in (559 mm) wheels, but since 2002 some models have been available with 29 in (622 mm) wheels, which is the same diameter most commonly used for road bikes (also known as 700c).

Some dirt jumping bikes also have 24in wheels. (The name given to a rim diameter in inches is different from the rim's actual size.).