A mountain bike or mountain bicycle (abbreviated MTB) is a bicycle created for off-road cycling. This activity includes traversing of rocks and washouts, and steep declines, on dirt trails, logging roads, and other unpaved environments—activities usually called mountain biking. These bicycles need to be able to withstand the stresses of off-road use with obstacles such as logs and rocks. Wheels used on mountain bikes usually use wide, knobby tires for good traction on uneven terrain and shock absorption. Since the mid-1990s, front wheel suspension has become the norm and since the late 1990s, full front and rear suspension has become increasingly common. Some mountain bikes are also fitted with bar ends on the handlebars to give extra leverage for hill-climbing.

Since the development of the sport in the 1970s many new subtypes of mountain biking have developed, such as cross-country (XC) biking, all-day endurance biking, Freeride-biking, downhill mountain biking, and a variety of track and slalom competitions. Each of these place different demands on the bike requiring different designs for optimal performance. MTB development has included an increase in gearing, up to 30 speeds, to facilitate both climbing and rapid descents. However, recently the "1 by 10" trend has emerged, simplifying the gearing to one sprocket in the front and 10 in the rear of the drive train. This allows for lighter component weights while still maintaining a large spread of gearing options. Single speed mountain bikes are also becoming more and more popular. Other developments include disc instead of rim brakes and 29" tires instead of the traditional 26" tires.