In the summer, sanderlings have tan and brown feathers, mottled with darker patches. In the winter, they can be quite pale—light gray above, and white below—and often blend in with the sandy beach. They have long, black legs and long beaks. 

Small plump birds, sanderlings are smaller than a red knot but bigger than a semipalmated plover. Adults are 7 to 8 inches (18–20 centimeters) long with a wingspan of about 14 inches (36 centimeters). 

They nest on a variety of Arctic tundra habitats ranging from dry gravelly or rocky ridge sites to vegetated moist sites. In the winter, they are most prevalent on sandy beaches but also use lakeshores, tide pools, estuaries and mudflats. Inland populations may also be found near reservoirs, ponds, and shallow lakes, especially in the Canadian prairies.

North American breeding populations nest in the summer on high Arctic tundra, primarily in Greenland, and Canada, but also rarely in Alaska. Birds from Greenland mainly travel to West Africa. In the Western Hemisphere, wintering birds are distributed on beaches throughout the Americas (including as far north as southern Alaska and the Canadian Maritimes), but are relatively rare on the Gulf of Mexico. Globally, they breed also in Norway and Russia and are very widespread near the coasts of every continent in the world except Antarctica.

The oldest known sanderling was at least 13 years old and lived in Nova Scotia.

During the breeding season, mating pairs communicate by croaking, trilling and buzzing. During the non-breeding season, feeding flocks make a squeaky chatter of “wick-wick” sounds. When driving off predators, they will snarl and squawk.

Sanderlings run along shorelines, pecking at the wet sand left when waves recede. They typically eat invertebrates, including crabs, insects, worms, small bivalves and horseshoe crab eggs, but will also eat seeds, berries, roots, algae, moss and small plants. They eat mainly insects and plants during the breeding grounds, and mainly seafood during the winter. 

They sleep while incubating, with their bill tucked under their scapular feathers (the shoulder feathers over the wing). In non-breeding season, they will roost in large, closely packed flocks usually above the high tide.

Sanderlings are highly social. They are often found in small flocks with other sanderlings but sometimes with other small shorebirds. To avoid predators like peregrine falcon, they will congregate in a larger, tightly-packed flock that swoops and darts. 

Only birds who plan to breed return to the breeding grounds in the Arctic. Other birds save energy by avoiding the journey of thousands of miles. Sanderlings are usually monogamous, though some birds of both sexes have been observed mating with multiple partners.

Females build the nests without help from males, digging a small shallow depression, and lining it with a few plants and mosses. Mothers lay an average of four eggs in each brood, and both parents incubate them for about a month (23-32 days). One parent usually leaves the nest after the chicks hatch, while the attending parent remains with the chicks until they are fledged. Sometimes both parents stay. The chicks run after their parents after hatching. They learn from their parents and are independent at about three weeks old.

Sanderling populations are declining. Effects of climate change are predicted to effect high-latitude habitats where sanderlings nest. During the wintering season, human disturbance on beaches (including increased vehicle traffic), coastal development, and contaminants and oil spills are major threats.