The Altai Mountains, also spelled Altay Mountains, are a mountain range in Central Asia and Eastern Asia, where Russia, China, Mongolia, and Kazakhstan converge, and where the rivers Irtysh and Ob have their headwaters. The massif merges with the Sayan Mountains in the northeast, and gradually becomes lower in the southeast, where it merges into the high plateau of the Gobi Desert. It spans from about 45° to 52° N and from about 84° to 99° E.

A mountain range or hill range is a series of mountains or hills arranged in a line and connected by high ground. A mountain system or mountain belt is a group of mountain ranges with similarity in form, structure, and alignment that have arisen from the same cause, usually an orogeny. Mountain ranges are formed by a variety of geological processes, but most of the significant ones on Earth are the result of plate tectonics. Mountain ranges are also found on many planetary mass objects in the Solar System and are likely a feature of most terrestrial planets.