Tresus is a genus of saltwater clams, marine bivalve mollusks in the family Mactridae.

They prefer sand, mud, and gravel substrates, normally burying themselves 12–16 inches (30–41 cm), so they are much easier to dig than geoducks.

Tresus clams often have a relationship with small commensal pea crabs, often a mating pair, which enter through the large siphon and live within the mantle cavity of the horse clam.

The meat is good and makes excellent chowder.

They tend to be ignored by sport diggers in Washington but not in Oregon.