An amebocyte or amoebocyte is a mobile cell (moving like an amoeba) in the body of invertebrates such as echinoderms, mollusks or sponges. Similar to some of the white blood cells of vertebrates, in many species amebocytes are found in the blood or body fluid and play a role in the defense of the organism against pathogens.

Limulus amebocyte lysate, an aequous extract of amebocytes from the horseshoe crab (Limulus polyphemus), is commonly used in a test to detect bacterial endotoxins.

In sponges, amebocytes or archaeocytes are cells found in the mesohyl that can transform into any of the animal's more specialized cell types.

In old literature, the term "amebocyte" is sometimes used as a synonym of "phagocyte".