The Apicomplexa (also called Apicomplexia) are a large phylum of parasitic alveolates. Most of them possess a unique form of organelle that comprises a type of non-photosynthetic plastid called an apicoplast, and an apical complex structure. The organelle is an adaptation that the apicomplexan applies in penetration of a host cell.

The Apicomplexa are unicellular and spore-forming. All species are obligate endoparasites of animals, except Nephromyces, a symbiont in marine animals, originally classified as a chytrid fungus. Motile structures such as flagella or pseudopods are present only in certain gamete stages.

The Apicomplexa are a diverse group that includes organisms such as the coccidia, gregarines, piroplasms, haemogregarines, and plasmodia. Diseases caused by Apicomplexa include:

• Babesiosis (Babesia)
• Malaria (Plasmodium)
• Cryptosporidiosis (Cryptosporidium parvum)
• Cyclosporiasis (Cyclospora cayetanensis)
• Cystoisosporiasis (Cystoisospora belli (formerly known as "Isospora Belli"))
• Toxoplasmosis (Toxoplasma gondii)

The name of the taxon Apicomplexa derives from two Latin words—apex (top) and complexus (infolds)—and refers to a set of organelles in the sporozoite. The Apicomplexa comprise the bulk of what used to be called the Sporozoa, a group of parasitic protozoans, in general without flagella, cilia, or pseudopods. Most of the Apicomplexa are motile, however, by use of a gliding mechanism that uses adhesions and small static myosin motors. The other main lines were the Ascetosporea (now in Rhizaria), the Myxozoa (now known to be highly derived cnidarian animals), and the Microsporidia (now known to be derived from fungi). Sometimes, the name Sporozoa is taken as a synonym for the Apicomplexa, or occasionally as a subset.