Amoeba, or Ameba, a protozoan (one-celled organism) noted for the simplicity of its body structure. Amoebas (or amoebae) are widely distributed over the world in fresh and salt water, damp soil, moist vegetation, and the bodies of animals. Most of the amoebas that live in animal bodies do no apparent harm, but one species causes amoebic dysentery, a serious and often fatal disease. Most amoebas are of microscopic size, but the two species known as the common amoeba and giant amoeba can sometimes be seen with the unaided eye. Living amoebas—especially the common amoeba—are often studied in biology classes because they are easily obtained, and because they are easily observed through a microscope.

The amoeba is a jellylike mass of protoplasm (living matter) that is constantly changing its shape. A cell membrane encloses a thin outer layer of transparent protoplasm called the ectoplasm and a granular inner mass called the endoplasm. The endoplasm contains the cell nucleus, which controls the amoeba's life processes. In most species, the endoplasm also contains a water bubble, called the contractile vacuole, that regulates the amoeba's water content. Within the endoplasm there are also bits of food enclosed in food vacuoles.

The amoeba does not have permanent body parts to perform its various functions. It uses temporary extensions of its body to move from place to place and to capture food. These temporary extensions are called pseudopodia. The amoeba's food includes other protozoans, rotifers, and microscopic algae and roundworms. It obtains these organisms by extending pseudopodia to surround them; the organisms are then digested within the food vacuoles. Body wastes are excreted by diffusion through the cell membrane and by means of the contractile vacuole. Most amoebas reproduce by fission.

Some biologists consider amoebas to make up a class, called Sarcodina by some and Rhizopoda by others, of the phylum Protozoa; others place them in their own phylum, also called Sarcodina or Rhizopoda. The common amoeba is Amoeba proteus; the giant amoeba, Pelomyxa carolinensis. The amoeba that causes dysentery is Entamoeba histolytica.