Telonemia is a phylum of microscopic eukaryotes commonly known as telonemids. They are unicellular free-living flagellates with a unique combination of cell structures, including a highly complex cytoskeleton unseen in other eukaryotes.

Telonemia shares several distinctive features with its related group, the SAR supergroup. Among these features are cortical alveoli, small sacs beneath the cell's surface that act as cushions, providing support and helping to maintain the cell's shape. Additionally, they possess tripartite mastigonemes, complex three-part hair-like structures on their flagella, the whip-like tails used for movement. These structures enhance their swimming capabilities by increasing resistance against water. Furthermore, Telonemia is equipped with filopodia, very thin, thread-like projections extending from the cell body. These projections can serve various purposes, such as aiding in movement or capturing food particles by wrapping around them. Together, the two lineages compose the TSAR clade.

This phylum is monotypic, composed of a single class Telonemea, order Telonemida and family Telonemidae. It is classified in three genera and seven species, although numerous undescribed clades of environmental DNA are known. They are detected in all marine and freshwater environments, where they prey on bacteria and small phytoplankton by engulfing them in their plasma membrane (phagotrophy).

The phylum Telonemia comprises microscopic unicellular eukaryotes, or protists. Most of the diversity of telonemids is morphologically uncharacterized. The few described species are free-living predatory phagotrophic flagellates composed of pear-shaped cells with two flagella. These cells measure approximately 5–10 μm in length and 3–7 μm in width. The flagella have different lengths, with the short one measuring up to 12 μm and the long one measuring up to 16 μm. Between the flagella protrudes a short proboscis-like structure, known as a rostrum. Their mitochondrial cristae are tubular. They have a unique multi-layered cytoskeleton of high complexity, composed of layers of microfilaments and microtubules, unseen in any other eukaryote. They exhibit a unique combination of cell traits that were previously believed to be exclusive to different chromalveolate groups, such as complex tripartite mastigonemes (as in stramenopiles), cortical alveoli-like structures (as in alveolates) and filopodia (as in rhizarians). Despite their evolutionary proximity to chromalveolates, they lack chloroplasts.

Telonemids feed on a wide range of organisms, namely bacteria and phytoplankton ranging in size between pico- and nanoplankton. They are widely distributed and are sometimes abundant, implying they may play an important ecological role in aquatic ecosystems. Around one hundred clades of environmental sequences from undescribed telonemids have been recovered in a variety of marine locations (Antarctic, Arctic and Indian Oceans; Mediterranean, Baltic, Kara, Marmara and White seas), including deep sea, and freshwater bodies from different regions (Norway, France, Antarctica, Finland, Canada, Japan). Several telonemid clades favor open waters with lower nutrients, such as the Canada Basin and offshore the Mackenzie River, suggesting that they are able to thrive in low-productivity ecosystems (i.e. oligotrophic).

The first telonemid genus and species, Telonema subtile, was described by Karl Griessmann in 1913 from crude cultures of the green alga Ulva and of red algae off the coast of Roscoff and Naples. Eighty years later, in 1993, American protistologist Thomas Cavalier-Smith created a family Telonemidae, order Telonemida and class Telonemea to contain this protist. Initially, this group was included within the now obsolete phylum Opalozoa, along with other unrelated groups of flagellates such as apusomonads, jakobids, cercomonads, spongomonads, katablepharids, ebriids, proteomyxids and so on. In this scheme, the class Telonemea was distinguished by the presence of two posterior cilia of equal length (isokont cilia). It contained an additional order besides Telonemida, Nephromycida, which comprised the genus Nephromyces (later treated as an apicomplexan). In 2005 a second species of telonemid was described, T. antarcticum, from the surface waters of the Oslofjord.

Since 2006, Telonemea was separated into a new eukaryotic phylum Telonemia by protistologist Kamran Shalchian-Tabrizi and coauthors, on the basis of phylogenetic analyses that placed it near chromalveolate groups such as Haptista and Cryptista. However, in 2015, Cavalier-Smith and coauthors rejected their treatment as an independent phylum and transferred Telonemea to the phylum Cryptista, under the obsolete subphylum Corbihelia. This subphylum included other protists with a pharyngeal basket or radiating axopodia such as Picomonas (later classified as a separate phylum Picozoa closely related to red algae) and Microheliella (now proposed as the sister group to Cryptista). In addition, they transferred T. antarcticum to a new genus Lateronema, on the basis of phylogenetic distance from Telonema.

Numerous phylogenetic analyses in the following years solidified the position of Telonemia as the sister clade to the SAR supergroup, both collectively composing the TSAR clade (Telonemia, Stramenopila, Alveolata and Rhizaria), which lead Cavalier-Smith to finally consider Telonemia a separate phylum in 2022. In the same year, five more species and a third genus, Arpakorses, were described by protistologist Denis Victorovich Tikhonenkov and coauthors.