Acanthamoeba is a genus of amoebae, one of the most common protozoa in soil, and also frequently found in freshwater and other habitats. The cells are small, usually 15 to 35 μm in length and oval to triangular in shape when moving. The pseudopods form a clear hemispherical lobe at the anterior, and there are various short filose extensions from the margins of the body. These give it a spiny appearance, which is what the name Acanthamoeba refers to. Cysts are common. Most species are free-living bacterivores, but some are opportunists that can cause infections in humans and other animals.

Diseases caused by Acanthamoeba include amoebic keratitis and encephalitis. The latter is caused by Acanthamoeba entering cuts and spreading to the central nervous system. The former is a rare disease where amoebae invade the cornea of the eye. It is nearly always associated with contact lens use, as Acanthamoeba can survive in the space between the lens and the eye. For this reason, contact lenses should be washed with special saline solutions and should be removed when swimming or surfing. To detect Acanthamoeba on a contact lens in a laboratory, a plain agar plate with a layer (a lawn) of E. coli is made. Part of the contact lens is placed on the agar plate. If Acanthamoeba are present, they will ingest the bacteria, leaving a clear patch on the plate around the area of the lens.

Species of Acanthamoeba are distinguished mainly on the form of cysts, and include the following; those marked with an asterisk are known to cause infections.

A. astronyxis
A. castellanii*
A. comandoni
A. culbertsoni*
A. divionensis
A. griffini
A. hatchetti*
A. healyi*
A. jacobsi
A. lenticulata
A. lugdunensis
A. mauritaniensis
A. palestinensis
A. pearcei
A. polyphaga*
A. pustulosa
A. quina
A. rhysodes
A. royreba*
A. terricola
A. triangularis
A. tubiashi