The serpent is a low-pitched early wind instrument in the lip-reed 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 or parchment. 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century, keys were added to improve intonation, and several upright variants were developed and used, until they were superseded first by the ophicleide and ultimately by the valved tuba.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 or timbre 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The instrument uses a mouthpiece about the same size as a trombone mouthpiece, originally made from ivory, horn or hardwood. The cup profiles of most historical serpent and bass horn mouthpieces were a distinctly hemispherical or ovate bowl shape, with a sharper-edged and narrower "throat" at the bottom of the cup than modern trombone mouthpieces, before expanding more-or-less conically through the shank. The mouthpiece 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 by dampening resonances they nevertheless aid note selection and contribute to the serpent's characteristic soft timbre. 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 remarked that Berger's instruments are much improved, finally allowing players to approach the serpent "in partnership rather than in combat". A modern basson russe is made by French instrument maker Jérôme Wiss.

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; French: serpenteau) 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.