With their 'smiling', whiskered faces and relatively small heads, the large Weddell seals are quite appealing in appearance. After molting the adults' bodies are blue-black and spotted silver-grey, then fading to rust-brown. Their short, dense fur protects them from water temperatures as low as 28°F. Weddell seals have whiskers (called vibrissae) which enhance their sense of touch. V-shaped nostrils close when the animal is at rest. Unlike crabeater and Leopard seals, Weddell seals are somewhat fat and not so streamlined. Females are slightly larger than males. Since Weddells use their strong teeth to chew and scrape breathing holes in the ice, extensive tooth wear occurs and is often a cause of mortality as older seals lose the ability to hunt or maintain their holes. Both male and female Weddell seals vocalize noisily underwater using a variety of calls; males may do so to maintain established territories.

Feeding mainly at night, Weddells dive to impressive depths in pursuit of fish, squid, and krill. By collapsing their lungs and lowering their breathing rates, they are able to stay underwater for up to an hour. In addition, Weddells have been observed blowing air bubbles into cracks under the sea ice to flush out prey. Like all seals, Weddells don't chew their food; they swallow it in large chunks underwater.