The serpent is a low-pitched early wind instrument in the brass family developed in the Renaissance era. It has a trombone-like mouthpiece, with six tone holes arranged in two groups of three fingered by each hand. It is named for its long, conical bore bent into a snakelike shape, and unlike most brass instruments is made from wood with an outer covering of leather. A distant ancestor of the tuba, the serpent is related to the cornett and was used for bass parts from the 17th to the early 19th centuries.

In the early 19th century, keys were added to improve intonation, and several upright variants were developed and used, until they were superseded first by the ophicleide and ultimately by the valved tuba. After almost entirely disappearing from orchestras, the serpent experienced a renewed interest in historically informed performance practice in the mid-20th century. Several contemporary works have been commissioned and composed, and serpents are again made by a small number of contemporary manufacturers. The sound of a serpent is somewhere between a bassoon and a euphonium, and it is typically played in a seated position, with the instrument resting upright between the player's knees.

Although closely related to the cornett, the serpent has thinner walls, a more conical bore, and no thumb-hole. The original serpent was typically built from hardwood, usually walnut or other tonewoods like maple, cherry, or pear, or sometimes softer woods like poplar. In France, the instrument was made from bonding two double-S-shaped halves, each carved from a single large piece of wood. In England, it was usually made from several smaller curved tubular wooden segments, each made by gluing two hollowed halves together. The whole instrument was then glued and bound with an outer covering of leather. A small number were made instead from copper or brass; one brass serpent in the Museo internazionale e biblioteca della musica in Bologna was built in 1773 with an added outer layer of leather.

The instrument uses a mouthpiece about the same size as a tenor trombone mouthpiece, originally made from ivory, horn or wood, which fits into the bocal or crook, a small length of brass tubing that emerges from the top wooden segment.

The serpent has six tone holes, in two groups of three, fingered by each hand. It is initially challenging to play the instrument with good intonation, due in large part to the positions of the tone holes. They are arranged mainly to be accessible to the player's fingers, rather than in acoustically correct positions, which for some of them would be out of reach. The lower tone holes are too small to act effectively to shorten the air column, but they dampen resonances and provide a mellow timbre suitable for accompanying the voice. While early serpents were keyless, later instruments added keys for additional holes out of reach of the fingers to improve intonation, and extend range. A mid-19th century model by London instrument maker Thomas Key has 14 keys, and survives in St Fagans National Museum of History in Cardiff, Wales.

Modern replicas are made by several specialist instrument makers, employing acoustic analysis and modern fabrication materials and techniques to further improve the serpent's intonation. Some of these techniques include use of modern composite materials and polymers, 3D printing, and changing the placement of tone holes. Swiss serpent maker Stephan Berger in collaboration with French jazz musician Michel Godard has developed an improved serpent based on studying well-preserved museum instruments, and also makes a lightweight model from carbon fibre. English serpent player and musicologist Clifford Bevan remarks that Berger's instruments are much improved, finally allowing players to approach the serpent "in partnership rather than in combat".

The majority of surviving specimens in museums and private collections are nominally bass serpents built in eight-foot (8′) C, thus having a total tubing length of about 8 feet (2.4 m). A few slightly smaller specimens were built in D, and military serpents could sometimes vary in pitch between D♭ and B♭.

The tenor serpent or serpet arose in the 1960s when English early music specialist and instrument maker Christopher Monk began his efforts to produce modern reproduction serpents by first building a half-sized pattern, which equated to a tenor size in 4′ C. Through a pantographic milling process, he scaled up the pattern by 2× to carve a bass serpent. Since he could just as easily carve at 1×, he was also able to produce tenor serpents popular in serpent ensembles, and usable by players with smaller fingers. The soprano serpent, or worm, was built by scaling the tenor serpent by 0.5× to produce an instrument in 2′ C, two octaves higher than the standard serpent. It first appeared in the 1980s, made as a novelty instrument by Monk. There is no repertoire or other evidence of the historical existence of these sizes.

The contrabass serpent, nicknamed the anaconda and built in 16′ C one octave below the serpent, was an English invention of the mid-19th century with no historical repertoire. The prototype instrument was built c. 1840 by Joseph and Richard Wood in Huddersfield as a double-sized English military serpent, and survives in the University of Edinburgh museum collection. Its use of keys, progressively larger tone holes, and an open top tone hole make it essentially a serpent-shaped contrabass ophicleide. During the serpent's modern revival, two more contrabass serpents were built in the original serpent ordinaire form in the 1990s by Christopher Monk's workshop, by doubling the pattern for a bass serpent. They were called "George" and "George II". The first, commissioned by musicologist and serpent player Philip Palmer, was owned by American trombonist and serpent player Douglas Yeo for a time and features in some of his serpent recordings. At least four other contrabass serpents have also been built: one from PVC piping in 1986, two from box plywood based on a "squarpent" design by American serpent player and curator Paul Schmidt, and one in 2014 from spare tuba and sousaphone parts.

There is little direct material or documentary evidence for the exact origin of the serpent. French historian Jean Lebeuf claimed in his 1743 work Mémoires Concernant l'Histoire Ecclésiastique et Civile d’Auxerre that the serpent was invented in 1590 by Edmé Guillaume, a clergyman in Auxerre, France, which is generally accepted. Some scholars propose that the serpent may have evolved from large, curved bass cornetts that were in use in Italy in the 16th century, but the lack of knowledge of the serpent in early 17th century Italy, or surviving early serpents outside of France, counts against this idea. The serpent was certainly used in France since the early 17th century, to strengthen the cantus firmus and bass voices of choirs in plainchant. This original traditional serpent was known as the serpent ordinaire or serpent d'église (lit. 'ordinary serpent' or 'church serpent'). Around the middle of the 18th century, the serpent began to appear in military bands, chamber ensembles, and later in orchestras.

Towards the end of the 18th century, the increased popularity of the serpent in military bands drove the subsequent development of the instrument to accommodate marching or mounted players. In England, a distinct military serpent was developed which had a more compact shape with tighter curves, added extra keys to improve its intonation, and metal braces between the bends to increase its rigidity and durability. In France around the same time several makers produced a serpent militaire initially developed by Piffault (by whose name they are also known) that arranges the tubing vertically with an upward turned bell, reminiscent of a tenor saxophone.