A protist (/ˈproʊtɪst/) is any eukaryotic organism (one with cells containing a nucleus) that is not an animal, plant, or fungus. The protists do not form a natural group, or clade, since they exclude certain eukaryotes with whom they share a common ancestor;^{[lower-alpha 1]} but, like algae or invertebrates, the grouping is used for convenience. In some systems of biological classification, such as the popular five-kingdom scheme proposed by Robert Whittaker in 1969, the protists make up a kingdom called Protista, composed of "organisms which are unicellular or unicellular-colonial and which form no tissues".^{[1][lower-alpha 2]} In the 21st century, the classification shifted toward a two-kingdom system of protists: Chromista (containing the chromalveolate, rhizarian and hacrobian groups) and Protozoa (containing excavates and all protists more closely related to animals and fungi).^[2]

The following groups contain protists. The clade Opisthokonta also contains the animals and the fungi, and the kingdom Archaeplastida also contains algae and plants.

Key to symbols used:

(P) = Paraphyletic group.

† = Extinct group.

? = Group of uncertain taxonomic position.

* = Group lacking molecular data.

[ ] = Synonyms and alternative names.

This article largely follows the efforts of the scientific community and the International Society of Protistologists to revise the taxonomy of protists in a manner that reflects their phylogeny and evolution, striving away from the use of historical paraphyletic taxa and relying exclusively on clades as the basis of the classification. The reference used for the general arrangement of groups is the publication Revisions to the Classification, Nomenclature and Diversity of Eukaryotes, published in 2019 by researcher Sina Adl and coauthors.^[3] Other studies are used for some specific groups of protists that have been revised after 2018, such as slime moulds,^[4] glaucophyte algae,^[5] arcellinid testate amoebae^[6] and planktonic foraminifera.^[7]

The following cladogram shows the relationships between the major groups of protists, as well as the kingdoms of animals, plants and fungi, according to the most recent phylogenetic analyses.^{[8][9][10][11][12]}

• Microheliella maris^{[lower-alpha 12]} Cavalier-Smith 2012
• Telonemia^{[lower-alpha 13]} Shalchian-Tabrizi 2006 [Telonemidae Cavalier-Smith 1993]. Arpakorses,^[35] Lateronema, Telonema.
• Provora Tikhonenkov et al. 2022^[10]
  • Nibbleridia Tikhonenkov et al. 2022. Nibbleromonas, Ubysseya.
  • Nebulidia Tikhonenkov et al. 2022. Nebulomonas, Ancoracysta.