A foraminifer is any protozoan belonging to the subclass Sarcodina, order Foraminiferida – amoeboid animals that live among plankton in the sea. They have multiply-chambered chalky shells (tests) which may be spiral, straight, or clustered, are made up of secreted calcite (rarely silica or aragonite) or of agglutinated particles, and vary in size from microscopic to 5 centimeters (2 inches) across, according to species. Many remain as fossils and are useful in geological dating.

When foraminifer die, their shells sink to the ocean floor to form large deposits, a source of chalk and limestone.