Similar in size to a sparrow, these small sandpipers have long black legs, black bills, a gray back and white breast with some gray streaking. 

Wingspan lengths are between 3.7 and 4 inches (9.3-10 centimeters). They can weigh anywhere from 17-51 grams (the weight of a single AA battery, or up to 2 AA batteries), depending on how much fat they have put on for migration.

During the summer breeding months, they range throughout subarctic and Arctic regions, seeking out nesting areas with low vegetation, especially near fresh water. When they migrate, they often stop along oceans, lakes and wetlands, and sometimes take advantage of small pools of water in farm fields.

The oldest known semipalmated sandpiper was 16 years old.

Their usual call is loud "cherk" sound. They use different vocalizations when defending their territories, protecting chicks, or responding to predators. Males make a sound like a motorboat during aerial displays when trying to attract a mate.

They eat mainly invertebrates, including mollusks, polychaete worms, fly larvae and tiny crustaceans. Some will eat biofilm, a gelatinous goo of microalgae and tiny invertebrates, on mudflats.

Semipalmated sandpipers are active during the daytime. Some individuals will forage during the night. 

Semipalmated sandpipers join mixed flocks of other shorebirds during migration and wintering. They can defend feeding areas and show aggression towards other shorebirds. During the breeding season, they are generally not social and will defend territories.  Males arrive in the summer breeding grounds earlier than the females, and jockey with each other for territories, though they tend to prefer the same territories as they had the year before. 

They are socially monogamous throughout the breeding season. They typically nest near water on top of small hummocks or ridges or in low tundra.

Males prepare shallow scrapes in the gravel or sand, and females choose one that she then lines with whatever is nearby including leaves, grass or moss.

The female lays one clutch of four pale speckled eggs, colored to blend in to the surroundings. Both parents incubate the eggs. The female usually leaves one to 11 days after the eggs hatch, and the male guards the young for the next week or two before they are able to look after themselves.

Semipalmated sandpiper numbers decreased because of unrestricted hunting in the late 1800s, but this pattern reversed with protection of migratory birds through the Migratory Bird Treaty Act in 1916. Unfortunately, since the 1970's, semipalmated sandpipers and other shorebird populations have declined rapidly. Threats include habitat loss (especially along coasts), human disturbance and climate change.