Adult male Weddell Seals reach 2.9 m in length, while females reach 3.3 m. Adults in their prime weigh 400-450 kg, with females being somewhat heavier than males, sometimes reaching over 500 kg (Stirling 1971). Adult female weight fluctuates dramatically during the year with significant weight loss occurring after birth and during lactation. Newborns are about 1.5 m long and average 29 kg. Females become mature at three to six years of age and males at seven to eight years. The annual pregnancy rate of mature females is 70-80%. Gestation lasts 11 months, including a delay of implantation of two months. Longevity is approximately 25 years (Stirling 1971, Reijnders et al. 1990).

Weddell Seals breed in areas of predictable stable fast ice or on land, allowing them to form loose aggregations (or breeding colonies) at specific sites (Siniff 1981, Testa et al. 1990). Weddell Seal pups are born from October through November. Adult females nurse their pups for a prolonged period (seven to eight weeks), and although they fast during the first one to two weeks, adult females do forage during lactation (Stirling 1971). Pups are born earlier at lower latitudes than at higher latitudes (Testa et al. 1990). Females enter oestrus approximately one week before weaning their pup, and copulation occurs underwater, where males maintain territories by controlling access to breathing holes and cracks. Adult Weddell Seals display strong site fidelity, with both males and females returning to the same breeding colony. When in the shore-fast ice habitat, Weddell Seals tend to congregate in loose groups along recurrent cracks, leads, and near access holes to the water (Siniff 1981). The behaviour of animals breeding in the pack ice or in the Sub-Antarctic islands is not well known.

Satellite data on movement patterns of Weddell Seals have increased during the last decade, providing a clearer picture of their distribution patterns. Weddell Seals habitat utilization and habitat patterns vary largely at a regional scale, showing large differences in the scale of their movements (tens to hundreds of km) depending on the area they inhabit (K. T. Goetz pers. comm., Heerah et al. 2013). Further, there seems to be large individual variability in their patterns of habitat usage, with some individuals staying in close proximity to their breeding colonies (e.g., McMurdo Sound), whereas others venture into the pack ice, likely exploiting polynyas and areas of thinner sea ice. The data available on newly weaned and subadult animals seem to indicate that younger animals move north from the continent and spend the winter in the pack ice.

The diving behaviour of Weddell Seals has been well studied, particularly at McMurdo Sound. They can reach depths of over 600 m, and can undertake dives of at least 82 minutes, feeding primarily feed at depths of 100–350 m, with a diurnal feeding pattern (Testa 1994). Weddell Seals are generalist predators, and their diet likely varies at a regional scale. Their diet primarily consists of notothenoid fish (Icefishes), particularly the Antarctic Silverfish which largely dominates in the diet in certain areas (Burns et al. 1998), but also includes Antarctic Toothfish, Myctophids and Cephalopods. Although Weddell Seals in fast ice areas are relatively protected, animals in the pack ice are vulnerable to predation by Killer Whales and Leopard Seals (Stirling 1969a, Visser et al. 2008).

Weddell Seals that remain in fast ice areas 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 (Stirling 1969ab). Such behaviour, though, comes at a cost, as Seals living in areas where extensive grinding of ice is necessary have accelerated wearing down of their teeth and decreased life expectancy (Stirling 1969b).