The smooth newt, European newt, northern smooth newt or common newt (Lissotriton vulgaris) is a species of newt. It is widespread in Europe and parts of Asia, and has been introduced into Australia. Individuals are brown with a spotted underside that ranges in colour from orange to white. They reach an average length of 8–11 cm (3.1–4.3 in); males are larger than females. The newts' skins are dry and velvety when they are living on land, but become smooth when they migrate into the water to breed. Males develop a more vivid colour pattern and a conspicuous skin seam (crest) on their back when breeding.

The smooth newt was originally described by Carl Linnaeus as a lizard, and was then given different genus names before the adoption of its current classification as a member of Lissotriton. There are currently three accepted subspecies of smooth newt. Formerly, there were also four subspecies—all with more restricted ranges—that are now classified as separate species, because they have been found to be distinct genetically as well as in appearance: the Caucasian smooth newt, the Greek smooth newt, Kosswig's smooth newt and Schmidtler's smooth newt. Together with these four species and the Carpathian newt, the smooth newt forms what is known as a species complex: some of the species hybridise with each other.

For most of the year, smooth newts live on land, are mostly nocturnal, and hide during the day. They can adapt to a wide range of natural or semi-natural habitats, from forests at field edges to parks and gardens. They feed mainly on invertebrates such as insects and earthworms and are themselves eaten mainly by fish, birds and snakes. Between spring and summer, they breed in ponds or similar bodies of water. Males court females with a ritualised underwater display. Females lay their eggs on water plants, and larvae hatch after 10 to 20 days. The larvae develop for around three months before metamorphosing into terrestrial juveniles, at which point they become known as efts. They reach maturity after two to three years, and the adults live for up to 14 years.

The smooth newt is abundant over much of its range and is classified as a species of least concern by the International Union for Conservation of Nature (IUCN). It has been negatively affected by habitat destruction and fragmentation and by the introduction of new species of fish. Like other European amphibians, the smooth newt has now been listed as a protected species by the Berne Convention.

Swedish naturalist Carl Linnaeus described the smooth newt in 1758 as Lacerta vulgaris, placing it in the same genus as the green lizards. It was later re-described under several different species and genus names, including Triton, Molge, Salamandra and Lissotriton, with in total 48 species synonyms published. Most recently, it was included in the genus Triturus, along with most European newts. This genus was found to be polyphyletic, containing several unrelated lineages, and the small-bodied newts, including the smooth newt, were therefore split off as separate genus in 2004 by García-París and colleagues. They used the name Lissotriton, introduced by the English zoologist Thomas Bell in 1839 with the smooth newt as type species but then considered a synonym of Triturus. "Lissotriton" is a combination of the Greek λισσός (lissós), meaning "smooth", and the name of Triton, an ancient Greek god of the sea, while the species epithet vulgaris means "common" in Latin.

Three subspecies are accepted by Pabijan, Wielstra and colleagues: L. v. vulgaris, L. v. ampelensis and L. v. meridionalis. These authors, followed by Amphibian Species of the World, recognise four former subspecies from southern Europe and west Asia as separate species, as they are morphologically and genetically distinct: the Greek smooth newt (L. graecus), Kosswig's smooth newt (L. kosswigi), the Caucasian smooth newt (L. lantzi) and Schmidtler's smooth newt (L. schmidtleri). The five smooth newt species and the Carpathian newt (L. montadoni), which is their sister species, have collectively been referred to as the "smooth newt species complex".

To distinguish the smooth newt from its close relatives, the English name "northern smooth newt" has been suggested. Other common names that have been used in the literature include: common newt, great water-newt, common water-newt, warty eft, water eft, common smooth newt, small newt, small eft, small evet, and brown eft.

Molecular phylogenetic analyses have shown that the smooth newt is distinct from its four close relatives – the Caucasian, Greek, Kosswig's, and Schmidtler's smooth newt – which were formerly considered subspecies (see section Taxonomy above). The relationships within this species complex have not been fully resolved. Within the smooth newt itself, genetic groups do not completely match the currently accepted subspecies (ampelensis, meridionalis, vulgaris), described based on morphology. The five smooth newt species collectively were estimated to have diverged from the Carpathian newt around four to six million years ago.