The mule deer (Odocoileus hemionus) is a deer whose habitat is in the western half of North America. It gets its name from its large mule-like ears. Its closest relative is the black-tailed deer (considered a subspecies of mule deer). Unlike its cousin, the white-tailed deer, mule deer are generally more associated with the land west of the Missouri River. The most noticeable differences between whitetails and muleys are the color of their tails and antlers. The mule deer's tail is black tipped. Mule deer antlers "fork" as they grow rather than branching from a single main beam (as with white-tails). Each year a buck's antlers start to grow in spring and are shed after mating season from mid-January to mid-April. Mule deer bucks have somewhat more prominent ears than females.

The mule deer is the largest of the Odocoileus genus, standing, on the average, 40 to 42 inches at the shoulders and stretching 80 inches or so nose to tail. An adult buck will weigh from 150 to 300 pounds on the hoof, with does averaging 100 to 175 pounds. With the occasional trophy-sized mule deer buck may weigh in around 450 pounds.

Instead of running, mule deer move with a bounding leap (stotting) with all four feet coming down together. Adult male mule deer are called bucks, adult females are called does, and young of both sexes are called fawns.

In addition to movements related to available shelter and food, the breeding cycle is important in understanding deer behavior. The "rut" or mating season usually begins in the fall as does go into estrus for a period of a few days and males become more aggressive, competing for mates. Does may mate with more than one buck and go back into estrus within a month if they do not mate. The gestation period is approximately 190–200 days, with fawns born in the spring, staying with their mothers during the summer and being weaned in the fall after approximately 60–75 days. A buck's antlers fall off during the winter, to grow again in preparation for the next season's rut. For more information see main article on deer.

In summer, it chiefly forages on not only herbaceous plants, but also various berries (including blackberry, huckleberry, salal, and thimbleberry). In winter, it forages on conifers (especially twigs of Douglas fir, cedar, Taxus yews, aspen, willow, dogwood, serviceberry, juniper, and sage). Year-round, it eats acorns and apples.

Mule deer prefer "edge" habitats and tend to move up or down with their preferred foods. Mule deer rarely travel far from water or forage, and tend to bed down within easy walking distance of both. Young mule deer tend to forage together in family groups while bucks tend to travel alone or with other bucks. Most actively foraging around dawn and dusk, they tend to bed down in protected areas mid-day, but will also forage at night in more open agricultural areas or when pressured by hunters. Repeated beds will often be scratched level, about the size of a washtub. Temporary beds will seem little more than flatened grass.