A protist (/ˈproʊtɪst/ PROH-tist) or protoctist is any eukaryotic organism that is not an animal, land plant, or fungus. Protists do not form a natural group, or clade, but are a paraphyletic grouping of all descendants of the last eukaryotic common ancestor excluding land plants, animals, and fungi.

Protists were historically regarded as a separate taxonomic kingdom known as Protista or Protoctista. With the advent of phylogenetic analysis and electron microscopy studies, the use of Protista as a formal taxon was gradually abandoned. In modern classifications, protists are spread across several eukaryotic clades called supergroups, such as Archaeplastida (photoautotrophs that includes land plants), SAR, Obazoa (which includes fungi and animals), Amoebozoa and "Excavata".

Protists represent an extremely large genetic and ecological diversity in all environments, including extreme habitats. Their diversity, larger than for all other eukaryotes, has only been discovered in recent decades through the study of environmental DNA and is still in the process of being fully described. They are present in all ecosystems as important components of the biogeochemical cycles and trophic webs. They exist abundantly and ubiquitously in a variety of mostly unicellular forms that evolved multiple times independently, such as free-living algae, amoebae and slime moulds, or as important parasites. Together, they compose an amount of biomass that doubles that of animals. They exhibit varied types of nutrition (such as phototrophy, phagotrophy or osmotrophy), sometimes combining them (in mixotrophy). They present unique adaptations not present in multicellular animals, fungi or land plants. The study of protists is termed protistology.

Protists are a diverse group of eukaryotes that are primarily single-celled and microscopic and exhibit a wide variety of shapes and life strategies. They have different life cycles, trophic levels, modes of locomotion, and cellular structures. Although most protists are unicellular, there is a considerable range of multicellularity amongst them; some form colonies or multicellular structures visible to the naked eye. The term 'protist' refers to all eukaryotes that are not animals, land plants or fungi, the three traditional eukaryotic kingdoms. Because of this definition by exclusion, protists compose a paraphyletic group that includes the ancestors of those three kingdoms.

The names of some protists (called ambiregnal protists), because of their mixture of traits similar to both animals and land plants or fungi (e.g., slime molds and flagellated algae like euglenids), have been published under either or both of the botanical (ICNafp) and the zoological (ICZN) codes of nomenclature.

Protists display a wide range of distinct morphological types that have been used to classify them for practical purposes, although most of these categories do not represent evolutionary cohesive lineages or clades and have instead evolved independently several times. The most recognizable types are:

The species diversity of protists is severely underestimated by traditional methods that differentiate species based on morphological characteristics. The number of described protist species is very low (ranging from 26,000 to over 76,000) in comparison to the diversity of land plants, animals and fungi, which are historically and biologically well-known and studied. The predicted number of species also varies greatly, ranging from 140,000 to 1,600,000, and in several groups the number of predicted species is arbitrarily doubled. Most of these predictions are highly subjective. Molecular techniques such as environmental DNA barcoding have revealed a vast diversity of undescribed protists that accounts for the majority of eukaryotic sequences or operational taxonomic units (OTUs), dwarfing those from land plants, animals and fungi. As such, it is considered that protists dominate eukaryotic diversity.

The evolutionary relationships of protists have been explained through molecular phylogenetics, the sequencing of entire genomes and transcriptomes, and electron microscopy studies of the flagellar apparatus and cytoskeleton. New major lineages of protists and novel biodiversity continue to be discovered, resulting in dramatic changes to the eukaryotic tree of life. The newest classification systems of eukaryotes do not recognize the formal taxonomic ranks (kingdom, phylum, class, order...) and instead only recognize clades of related organisms, making the classification more stable in the long term and easier to update. In this new cladistic scheme, the protists are divided into various branches informally named supergroups. Most photosynthetic eukaryotes fall under the Diaphoretickes clade, which contains the supergroups Archaeplastida (which includes land plants) and SAR (including, Stramenopiles, Alveolata and Rhizaria), as well as the phyla Telonemia, Cryptista and Haptista. The animals and fungi fall into the Amorphea supergroup, which contains the phylum Amoebozoa and several other protist lineages. Various groups of eukaryotes with primitive cell architecture are collectively known as the "Excavata".