The flugelhorn (/ˈfluːɡəlhɔːrn/), also spelled fluegelhorn, flugel horn, or flügelhorn, is a brass instrument that resembles the trumpet and cornet, but has a wider, more conical bore. Like trumpets and cornets, most flugelhorns are pitched in B♭, though some are in C. It is a type of valved bugle, developed in Germany in the early 19th century from a traditional English valveless bugle. The first version of a valved bugle was sold by Heinrich Stölzel in Berlin in 1828. The valved bugle provided Adolphe Sax (creator of the saxophone) with the inspiration for his B♭ soprano (contralto) saxhorns, on which the modern-day flugelhorn is modelled.

The German word Flügel means wing or flank in English. In early 18th century Germany, a ducal hunt leader known as a Flügelmeister blew the Flügelhorn, a large semicircular brass or silver valveless horn, to direct the wings of the hunt. Military use dates from the Seven Years' War, where this instrument was employed as a predecessor of the bugle.

The flugelhorn is generally pitched in B♭, like most trumpets and cornets. It usually has three piston valves and employs the same fingering system as other brass instruments, although four-valve versions and rotary-valve versions also exist. It can therefore be played by trumpet and cornet players, although it has different playing characteristics. The flugelhorn's mouthpiece is more deeply conical than either trumpet or cornet mouthpieces, but not as conical as a French horn mouthpiece.

Some modern flugelhorns feature a fourth valve that lowers the pitch by a perfect fourth (similar to the fourth valve on some euphoniums, tubas, and piccolo trumpets, or the trigger on trombones). This adds a useful low range that, coupled with the flugelhorn's dark sound, extends the instrument's abilities. Players can also use the fourth valve in place of the first and third valve combination (which is somewhat sharp).

A compact version of the rotary valve flugelhorn is the oval shaped kuhlohorn in B♭. It was developed for the German Protestant trombone choirs.

A pair of bass flugelhorns in C, called fiscorns, are played in the Catalan cobla bands that provide music for sardana dancers.

The tone is fatter and usually regarded as more mellow and dark than the trumpet or cornet. The sound of the flugelhorn has been described as halfway between a trumpet and a French horn, whereas the cornet's sound is halfway between a trumpet and a flugelhorn. The flugelhorn is as agile as the cornet but more difficult to control in the high register (from approximately written G₅), where in general it locks onto notes less easily.

The flugelhorn is a standard member of the British-style brass band, and it is also used frequently in jazz. It also appears occasionally in orchestral and concert band music. Famous orchestral works with flugelhorn include Igor Stravinsky's Threni, Ralph Vaughan Williams's Ninth Symphony, and Michael Tippett's third symphony. The flugelhorn is sometimes substituted for the post horn in Mahler's Third Symphony, and for the soprano Roman buccine in Ottorino Respighi's Pines of Rome. In HK Gruber's trumpet concerto Busking (2007) the soloist is directed to play a flugelhorn in the slow middle movement. The flugelhorn figured prominently in many of Burt Bacharach's 1960s pop song arrangements. It is featured in a solo role in Bert Kaempfert's 1962 recording of "That Happy Feeling". Flugelhorns have occasionally been used as the alto or low soprano voice in a drum and bugle corps.