Short-beaked echidnas are a medium-size monotreme mammal, rarely weighing more than 7 kg (15.4 pounds), covered on their back with stout spines among a fur coat of varying color from light brown to black. It is impossible to distinguish male from female echidnas by their appearance without picking them up to determine the presence or absence of a penis. They have a short, stubby tail. The hollow spines that cover most of their body are yellowish at the base and black at the tip and measure about 2 inches long. The underbelly lacks spines but is covered with fur and thick bristles. They have a long, tubular snout. Since they do not have teeth, they use their long, sticky tongue to gather food. There are two sets of hardened, keratinous spines (one set on the roof of the mouth and one at the base of the tongue) for grinding insect exoskeletons into a paste. Their feet have five flat claws that are adapted for digging, though their hindfeet are used primarily for grooming.

They are quadrupedal (moving on four legs) and able to run swiftly, climb well, and swim capably. They move about any time of the day or night. Their home ranges usually average 50 hectares (124 acres) and may overlap with the ranges of other individuals. To find shelter, they live under thick bushes, in hollow logs, under piles of debris, or in burrows. There is no specific nest site and no evidence of territorial behavior. For most of the year, they are solitary creatures.

Their snout is very important for sensing the environment. It provides them with a unique sense that uses electroreceptors. These mechano- and electroreceptors respond to small electric stimuli applied to the snout. The snout is also very important because it houses a very complex nasal passage that enables a good sense of smell, important for foraging, identifying prey, social interactions, and reading the environment. Vision is not their most important sense and they can do relatively well with impaired vision. Echidnas' external ears are actually quite large. However, they are hard to see because of their spines. They have the typical mammal inner ear arrangement (three bones called malleus, incus, and stapes). Their sense of hearing responds best to lower frequencies and may be ideal for hearing sounds made by food items, especially termites and ants.