Mountain biking is a cycling discipline that incorporates extreme elements such as navigating rocky terrain and cliffs, steep descents, muddy trails, and other uneven off-road paths. For this reason, the activity is commonly known as mountain bike riding or simply mountain biking.

The history of mountain bikes draws from traditions in the United Kingdom, particularly the Rough-Stuff Fellowship (established in 1955 as the world's oldest off-road cycling club), as well as early off-road races and tours across hilly European terrain. The term "mountain bike" (or "mountain bicycle") first appeared in print in 1966, credited to a rider in Oregon, USA, who built a rough-terrain trail bicycle. Early mountain bikes were modified heavy cruiser bicycles adapted for descending mountain trails. The modern sport gained significant popularity in the 1970s in Marin County, California, USA, where riders modified old single-speed balloon-tire bikes (known as "klunkers") for off-road use on trails around Mount Tamalpais. A 2006 documentary film titled Klunkerz: A Film About Mountain Bikes details this era, including early cyclocross-inspired rides and downhill races like Repack. By the late 1970s and early 1980s, bicycle companies began producing purpose-built mountain bikes using lightweight materials such as aluminum alloys (e.g., M4 aluminum). Although more expensive, materials like titanium, hydroformed aluminum, and carbon fiber frames have become increasingly common in recent years. The first mass-produced mountain bike was the Specialized Stumpjumper, introduced in 1981. From the 1990s to the 2000s, mountain biking evolved from a niche activity into a mainstream sport, featuring prominently in international competitions and World Championships. It debuted as an Olympic discipline (cross-country) in 1996.

In mountain biking, riders must execute the necessary maneuvers required by off-road mountain trails. The discipline utilizes specially designed mountain bikes built for durability, traction, and control on rough terrain. Competitions are typically held on hilly and mountainous routes featuring steep climbs and descents, sharp turns, and technical sections. Mountain biking encompasses several sub-disciplines (e.g., cross-country, downhill, enduro), each with specific formats, but the core objective is generally to complete the course in the fastest time while mastering varied terrain.