The smallest organisms found on Earth can be determined according to various aspects of organism size, including volume, mass, height, length, or genome size.
Given the incomplete nature of scientific knowledge, it is possible that the smallest organism is undiscovered. Furthermore, there is some debate over the definition of life, and what entities qualify as organisms; consequently the smallest known organism (microorganism) is debatable.
Many biologists consider viruses to be non-living because they lack a cellular structure and cannot metabolize by themselves, requiring a host cell to replicate and synthesize new products. A minority of scientists hold that, because viruses do have genetic material and can employ the metabolism of their host, they can be considered organisms. Also, an emerging concept that is gaining traction among some virologists is that of the virocell, in which the actual phenotype of a virus is the infected cell, and the virus particle is merely a reproductive or dispersal stage, much like pollen or a spore.
The smallest viruses in terms of genome size are single-stranded DNA (ssDNA) viruses. Perhaps the most famous is the bacteriophage Phi-X174 with a genome size of 5386 nucleotides. However, some ssDNA viruses can be even smaller. For example, Porcine circovirus type 1 has a genome of 1759 nucleotides and a capsid diameter of 17 nm. As a whole, the viral family geminiviridae is about 30 nm in length. However, the two capsids making up the virus are fused; divided, the capsids would be 15 nm in length. Other environmentally characterized ssDNA viruses such as CRESS DNA viruses, among others, can have genomes that are considerably less than 2,000 nucleotides.
The smallest RNA viruses in terms of genome size are small retroviruses such as rous sarcoma virus with genomes of 3.5 kilo base pairs (kb) and particle diameters of 80 nanometres (nm). The smallest double-stranded DNA viruses are the hepadnaviruses such as hepatitis B, at 3.2 kb and 42 nm; parvoviruses have smaller capsids, at 18-26 nm, but larger genomes, at 5 kb. It is important to consider other self-replicating genetic elements, such as satelliviruses, viroids and ribozymes.
The genome of Nasuia deltocephalinicola, a symbiont of the European pest leafhopper, Macrosteles quadripunctulatus, consists of a circular chromosome of 112,031 base pairs.
The genome of Nanoarchaeum equitans is 490,885 nucleotides long.
Pelagibacter ubique is one of the smallest known free-living bacteria, with a length of 370 to 890 nm and an average cell diameter of 120 to 200 nm. They also have the smallest free-living bacterium genome: 1.3 Mbp, 1354 protein genes, 35 RNA genes. They are one of the most common and smallest organisms in the ocean, with their total weight exceeding that of all fish in the sea.
Mycoplasma genitalium, a parasitic bacterium which lives in the primate bladder, waste disposal organs, genital, and respiratory tracts, is thought to be the smallest known organism capable of independent growth and reproduction. With a size of approximately 200 to 300 nm, M. genitalium is an ultramicrobacterium, smaller than other small bacteria, including rickettsia and chlamydia. However, the vast majority of bacterial strains have not been studied, and the marine ultramicrobacterium Sphingomonas sp. strain RB2256 is reported to have passed through a 220 nm ultrafilter. A complicating factor is nutrient-downsized bacteria, bacteria that become much smaller due to a lack of available nutrients.
Nanoarchaeum equitans is a species of microbe 200 to 500 nm in diameter. It was discovered in 2002 in a hydrothermal vent off the coast of Iceland by Karl Stetter. A thermophile that grows in near-boiling temperatures, Nanoarchaeum appears to be an obligatory symbiont on the archaeon Ignicoccus; it must be in contact with the host organism to survive.
Prasinophyte algae of the genus Ostreococcus are the smallest free-living eukaryote. The single cell of an Ostreococcus measures 0.8 μm across.
Several species of Myxozoa (obligately parasitic cnidarians) never grow larger than 20 µm. One of the smallest species (Myxobolus shekel) is no more than 8.5 µm when fully grown, making it the smallest known animal.
The shell of the nut clam Condylonucula maya grows 0.54 mm long.
The smallest water snail (of all snails) is Ammonicera minortalis in North America, originally described from Cuba. It measures 0.32 to 0.46 mm.
The smallest land snail is Acmella nana. Discovered in Borneo, and described in November 2015, it measures 0.7 mm. The previous record was that of Angustopila dominikae from China, which was reported in September 2015. This snail measures 0.86 mm.
The smallest crustacean, and indeed the smallest arthropod, is the tantulocarid Stygotantulus stocki, at a length of 94 μm (0.0037 in).
There is a debate between which spider is smallest. According to Guinness World Records, "Two contenders are from the [Symphytognathidae] genus Patu: males of Patu digua described in Colombia had a body length of 0.37 millimetres (0.015 inches), while the Samoan moss spider (P. marplesi) could be as small as 0.4 millimetres (0.016 inches) long." Other possible smallest spider species are the Frade cave spider known as Anapistula ataecina, and the dwarf orb weaver (Anapistula caecula), the females of which are 0.43 millimeters and 0.48 millimeters respectively. Males of both species are potentially smaller than the females, but no Anapistula ataecina or Anapistula caecula have been measured yet.
Cochlodispus minimus Mahunka has had an adult individual measured with a body length of 79 μm. However, PBS claims "The tiniest mite on record is 82 microns long" but does not name a species.
Adult males of the parasitic wasp Dicopomorpha echmepterygis can be as small as 139 μm long, smaller than some species of protozoa (single-cell creatures); females are 40% larger.
Megaphragma caribea from Guadeloupe, measuring 170 μm long, is another contender for smallest known insect in the world.
Beetles of the tribe Nanosellini are all less than 1 mm long; the smallest confirmed specimen is of Scydosella musawasensis at 325 μm long; a few other nanosellines are reportedly smaller, in historical literature, but none of these records have been confirmed using accurate modern tools. These are among the tiniest non-parasitic insects.
The western pygmy blue (Brephidium exilis) is one of the smallest butterflies in the world, with a wingspan of about 1 centimetre.
The smallest sea cucumber, and also the smallest echinoderm, is Psammothuria ganapati, a synaptid that lives between sand grains on the coast of India. Its maximum length is 4 mm. 
The smallest sea urchin, Echinocyamus scaber, has a test 6 mm across.
Patiriella parvivipara is the smallest starfish, at 5 mm across.
The smallest vertebrates (and smallest amphibians) known are Paedophryne amauensis frogs from Papua New Guinea, which range in length from 7.0–8.0 mm (0.28–0.31 in), and average 7.7 mm (0.30 in). Previously, the title of smallest vertebrate was held by members of the fish genus Paedocypris of Indonesia.
Main article: List of smallest fish
One of the smallest fish based on the minimum size at maturity is Paedocypris progenetica from Indonesia, with mature females measuring as little as 7.9 mm (0.31 in) in standard length. This fish, a member of the carp family, has a translucent body and a head unprotected by a skeleton.
One of smallest fish based on the minimum size at maturity is Schindleria brevipinguis from Australia, their females reach 7 mm (0.28 in) and males 6.5 mm (0.26 in), Males of S. brevipinguis have an average standard length of 7.7 mm (0.30 in); a gravid female was 8.4 mm (0.33 in). This fish, a member of the goby family, differs from similar members of the group in having its first anal fin ray further forward, under dorsal fin 4.
Male individuals of the anglerfish species Photocorynus spiniceps have been documented to be 6.2–7.3 mm (0.24–0.29 in) at maturity, and thus claimed to be a smaller species. However, these survive only by sexual parasitism and the female individuals reach the significantly larger size of 50.5 mm (1.99 in).
The average snout-to-vent length (SVL) of several specimens of the salamander Thorius arboreus was 17 mm (0.67 in).
Frogs include the smallest vertebrates known. The smallest known frog species is Paedophryne amauensis, with a snout-vent length reported as 7.7 mm, which occurs among leaf-litter in the tropical montane forests of New Guinea. Other very small frogs include Brachycephalus didactylus from Brazil (reported as 9.6–9.8 mm), several species of Eleutherodactylus such as E. iberia (around 10 mm) and E. limbatus (8.5–12 mm) and Eleutherodactylus orientalis (12.5 mm) from Cuba, Gardiner's Frog Sechellophryne gardineri from the Seychelles (up to 11 mm), several species of Stumpffia such as S. tridactyla (8.6–12 mm) and S. pygmaea (males 10–12.5 mm; females: 11 mm) and Wakea madinika (males: 11–13 mm; females: 15–16 mm) from Madagascar. Paedophryne swiftorum (body length 8.5 mm) is not included in the smallest vertebrates known with other nine species of frogs. The two species Microhyla borneensis (males:10.6–13 mm; females:16–19 mm) and Arthroleptella rugosa (males: 11.9–14.1 mm; females:15.5mm) were once the smallest known frogs from the Old World. In general these extremely small frogs occur in tropical forest and montane environments. There is relatively little data on size variation among individuals, growth from metamorphosis to adulthood or size variation among populations in these species. Additional studies and the discovery of further minute frog species are likely to change the rank order of this list.
Two geckos, the dwarf gecko (Sphaerodactylus ariasae) and the Virgin Islands dwarf sphaero (S. parthenopion), are the smallest known reptile species and smallest lizard, with a snout-vent length of 16 millimetres (0.63 in). A few Brookesia chameleons from Madagascar are equally small, with a reported snout-vent length of 15–18 mm for male dwarf chameleons (B. minima), 14–19 mm for male Mount d'Ambre leaf chameleons (B. tuberculata) and 15–16 mm for male B. micra, though females are larger. In 2021, a new species of Brookesia, B. nana, was discovered, with a snout-vent length of 13.5 mm, possibly making it the smallest known reptile.
Of the aforementioned geckos, S. ariasae was first described in 2001 by the biologists Blair Hedges and Richard Thomas. This dwarf gecko lives in Jaragua National Park in the Dominican Republic and on Beata Island (Isla Beata), off the southern coast of the Dominican Republic.
The smallest turtle is the speckled padloper tortoise (Homopus signatus) from South Africa. The males measure 6–8 cm (2.4–3.1 in), while females measure up to almost 10 cm (3.9 in).
The smallest crocodilian is the Cuvier's dwarf caiman (Paleosuchus palpebrosus) from northern and central South America. It reaches up to 1.6 m (5.2 ft) in length.
One of the smallest snakes known is the recently discovered Barbados threadsnake (Leptotyphlops carlae). Adults average about 10 cm (4 in) long, which is only about twice as long as the hatchlings. The Common blind snake (Indotyphlops braminus) measures 5.1–10.2 cm (2–4 in) long, occasionally up to 15 cm (6 in) long.
The smallest avian dinosaur is the bee hummingbird. The smallest known extinct dinosaur is Anchiornis, a genus of feathered dinosaur that lived in what is now China during the Late Jurassic Period 160 to 155 million years ago. Adult specimens range from 34 cm (13 in) long, and the weight has been estimated at up to 110 g (3.9 oz). Nevertheless, sizes of dinosaurs are commonly labelled with a level of uncertainty, as the available material often (or even usually) is incomplete.
With a mass of approximately 1.8 grams (0.063 oz) and a length of 5 centimetres (2.0 inches), the bee hummingbird (Mellisuga helenae) is the smallest bird species, the smallest warm-blooded vertebrate, and smallest known dinosaur. Called the zunzuncito in its native habitat on Cuba, it is lighter than a Canadian or U.S. penny. It is said that it is "more apt to be mistaken for a bee than a bird". The bee hummingbird eats half its total body mass and drinks eight times its total body mass each day. Its nest is 3 cm across.
The vulnerable Kitti's hog-nosed bat (Craseonycteris thonglongyai), also known as the bumblebee bat, from Thailand and Myanmar is the smallest mammal, at 3–4 centimetres (1.2–1.6 in) in length and 1.5–2 grams (0.053–0.071 oz) in weight.
The Etruscan shrew (Suncus etruscus), is the smallest mammal by mass, weighing about 1.8 g (0.063 oz) on average. The bumblebee bat has a smaller skull size. The smallest mammal that ever lived, the shrew-like Batodonoides vanhouteni, weighed 1.3 grams (0.046 oz).
The smallest known member of the rodent order is the Baluchistan pygmy jerboa, with an average body length of 4.4 cm (1.7 in).
The smallest member of the order Carnivora is the least weasel (Mustela nivalis), with an average body length of 114–260 mm (4.5–10.2 in). It weighs between 29.5 – 250 grams with females being lighter.
The smallest marsupial is the long-tailed planigale from Australia. It has a body length of 110–130 millimetres (4.3–5.1 in) (including tail) and weigh 4.3 grams (0.15 oz) on average.
The Pilbara ningaui is considered to be of similar size and weight.
The smallest member of the primate order is Madame Berthe's mouse lemur (Microcebus berthae), found in Madagascar, with an average body length of 92 mm (3.6 in).
The smallest cetacean, which is also (as of 2006) the most endangered, is the vaquita. Male vaquitas grow to an average of around 135 cm (53 in); the females are slightly longer, averaging about 141 cm (56 in) in length.
Duckweeds of the genus Wolffia are the smallest flowering plants. Fully grown, they measure 300 µm by 600 µm and reach a mass of just 150 µg.
Nanobes are thought by some scientists to be the smallest known organisms, about one tenth the size of the smallest known bacteria. Nanobes, tiny filamental structures first found in some rocks and sediments, were first described in 1996 by Philippa Uwins of the University of Queensland.