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Plasmodium, commonly known as the malaria parasite, is a large genus of parasitic protozoa. Infection with these protozoans is known as malaria, a deadly disease widespread in the tropics. The parasite always has two hosts in its life cycle: a mosquito vector and a vertebrate host.

The life-cycle is very complex, involving a sequence of different stages both in the vector and the host. These stages include sporozoites which are injected by the mosquito vector into the host's blood; latent hypnozoites which may rest undetected in the liver for up to 30 years; merosomes and merozoites which infect the red cells (erythrocytes) of the blood; trophozoites which grow in the red cells, and schizonts which divide there, producing more merozoites which leave to infect more red cells; and male and female sexual forms, gametocytes, which are taken up by other mosquitoes. In the mosquito's midgut, the gametocytes develop into gametes which fertilize each other to form motile zygotes which escape the gut, only to grow into new sporozoites which move to the mosquito's salivary glands, from where they are injected into the mosquito's next host, infecting it and restarting the cycle.

The genus Plasmodium was first described in 1885. It now contains about 200 species divided into several subgenera; as of 2006 the taxonomy was shifting, and species from other genera are likely to be added to Plasmodium. At least ten species infect humans; other species infect other animals, including birds, reptiles and rodents, while 29 species infect non-human primates. The parasite is thought to have originated from Dinoflagellates, photosynthetic protozoa.

The most common forms of human malaria are caused by Plasmodium falciparum, P. vivax, P. knowlesi, and P. malariae. P. falciparum malaria, common in sub-Saharan Africa, and P. knowlesi in South East Asia, are especially dangerous.