The Apicomplexa (also referred to as Apicomplexia) are a large group of protists, most of which possess a unique organelle called apicoplast and an apical complex structure involved in penetrating a host's cell. They are unicellular, spore-forming, and exclusively parasites of animals. Motile structures such as flagella or pseudopods are present only in certain gamete stages. This is a diverse group including organisms such as coccidia, gregarines, piroplasms, haemogregarines, and plasmodia. Diseases caused by apicomplexan organisms include, but are not limited to:

The name of the taxon Apicomplexa is derived from two Latin words - apex (top) and complexus (infolds) - and refers to a set of organelles in the sporozoite. The older taxon Sporozoa grouped the Apicomplexa together with the Microsporidia and Myxosporida. This grouping is no longer regarded as biologically valid and its use is discouraged.

The first apicomplexan protozoan was seen by Antony van Leeuwenhoek who in 1674 saw oocysts of Eimeria stiedae in the gall bladder of a rabbit. The first member of the phylum to be named (by Dufour in 1828) was Gregarina ovata in earwigs. Since then many more have been identified and named. During the quarter century 1826-1850, 41 species and 6 genera of Apicomplexa were named. In the quarter century 1951-1975, 1873 new species and 83 new genera were added.