The long-tailed marmot is a large, sturdy rodent weighing up to . Its typical weight range is from , with the lower weights in the spring directly after hibernation and the higher weights in the autumn just before hibernation where more than one–quarter of its mass can be fat. Males average slightly larger than females. Its head-and-body length is and the tail is about long. The tail is 37–55% of the head-and-body length. This is considerably longer than typical of other marmots, although the proportionally longest-tailed individuals of the grey (M. baibacina) and alpine marmots (M. marmota) are comparable to the shortest-tailed individuals of the long-tailed marmot. The eyes are close to the top of the rather-flattened head, the ears are small and the neck is short. The forelegs are longer than the hind legs.
