A boutonnière (French: [bu.tɔ.njɛʁ]) or buttonhole (British English) is a floral decoration, typically a single flower or bud, worn on the lapel of a tuxedo or suit jacket.

While worn frequently in the past, boutonnières are now usually reserved for special occasions for which formal wear is standard, such as at proms, homecomings, funerals, and weddings. (Women who wear jackets on these occasions may also wear boutonnières, but more typically a woman would wear a corsage.) Nowadays, a lapel pin is worn more often than flowers on business suits.

Traditionally, a boutonnière was worn pushed through the lapel buttonhole (on the left, the same side as a pocket handkerchief) and the stem is held in place with a loop at the back of the lapel. The flower's calyx, if pronounced such as those of a carnation, should be fully inserted into the buttonhole which would secure it tightly and flat against the lapel. Thus the buttonhole should ideally be at least 1⅛" long for there to be enough room to fit a standard sized flower's calyx. Otherwise, the calyx would not fit into the buttonhole and the flower head would hang freely and move about in the wind.

However, on many recently made coats and jackets, the lapel is made without the loop required, which would normally sit on the reverse of the lapel, beneath the buttonhole. Sometimes, the lapel buttonhole is in the "keyhole" shape, as opposed to the traditional straight cut, or is not even pierced through, in which case the boutonnière may be pinned onto the jacket lapel, although this may be considered unsightly and continued pinning could eventually damage the cloth or silk facing.