Adult males reach 2.9 m in length, while females reach 3.3 m. Adults in their prime weigh 400 to 450 kg, with females being somewhat heavier than males. Adult female weight fluctuates dramatically during the year with significant weight loss occurring after birth during lactation. Newborns are about 1.5 m long and average 29 kg. Females become mature at 3-6 years of age and males at 7-8 years. The annual pregnancy rate of mature females is 70-80%. Gestation lasts 11 months, including a delay of implantation of 2 months. Longevity is approximately 25 years (Reijnders et al. 1993).

Weddell seal pups are born from September through November and nursed for seven to eight weeks. Animals at lower latitudes pup earlier than animals living at higher latitudes. Males set up territories in the water around holes in the ice used by females to enter and leave the water. The only copulation that has been observed occurred underwater. The behavior of animals breeding in the pack ice or in the sub-Antarctic is not well known.

Weddell seals are not very social when out of the water, avoiding physical contact most of the time. However, they are loosely social; when in the shore-fast ice habitat they tend to congregate in groups along recurrent cracks, leads, and near access holes to the water. There is debate over whether or not this species is migratory or if there is just dispersal of weaned pups, juveniles and some older animals. Some individuals remain in residence year round in the fast ice at latitudes as high as 78°S in McMurdo Sound. Others, particularly newly weaned and subadult animals, move north from the continent and spend the winter in the pack ice. Seals living in fast ice areas or facing freezing over of access holes and leads, abrade and grind the ice to maintain access to and from the water. They bite at the ice and then rapidly swing the head from side to side to grind away the ice with their teeth. Seals living in areas where extensive grinding of ice is necessary for much of the year have accelerated wearing down of their teeth and decreased life expectancy. Predators include killer whales and leopard seals.

Weddell seals are prodigious divers, reaching over 600 m depth, and can undertake dives of at least 82 minutes. Deep dives are regularly used for foraging on Antarctic cod, and long dives occur when the animals are searching for new breathing holes, cracks and leads. In addition to Antarctic cod, the diet of Weddell seals primarily consists of Notothenid fishes. Squid and other invertebrates are also taken as small percentages of the diet.