"Rhiannon" is a song written by Stevie Nicks and originally recorded by Fleetwood Mac. First released on their self-titled album in 1975, it was subsequently issued as a single the following year.

"Rhiannon" was voted #488 in The 500 Greatest Songs of All Time by Rolling Stone magazine. Its US chart peak was in June 1976, when it hit #11. It reached #46 in the UK singles chart after re-release in February 1978.

The song is always referred to as simply "Rhiannon" on Fleetwood Mac albums. The suffixed title "Rhiannon (Will You Ever Win)" was only used on singles in certain territories.

Live performances of the song were sometimes prefaced with Nicks saying, "This is a song about an old Welsh witch." During 1975–1980, Fleetwood Mac's live performances of "Rhiannon" took on a theatrical intensity not present on the FM-radio single. The song built to a climax in which Nicks' vocals were so impassioned that, as drummer and band co-founder Mick Fleetwood said, "her Rhiannon in those days was like an exorcism."

Rhiannon is a prominent figure in Welsh mythology, mother to the Demetian hero Pryderi and wife to Pwyll (and later Manawydan fab Llyr). She is probably a reflex of the Celtic Great Queen goddess Rigantona and may also be associated with the horse goddess Epona.

She appears in both the first and third branches of the Mabinogi and is further mentioned in the early Arthurian prose tale Culhwch and Olwen.

Upon ascending the magical mound of Gorsedd Aberth, the Demetian king Pwyll witnesses the arrival of Rhiannon, appearing to them as a beautiful woman dressed in gold silk brocade and riding a shining white horse. Pwyll sends his best horsemen after her, but she always remains ahead of them, though her horse never does more than amble. After three days, he finally calls out to her, and Rhiannon tells him she has come seeking him because she would rather marry him than her fiancé, Gwawl ap Clud. A year after their meeting, Pwyll accidentally and foolishly promises Rhiannon to Gwawl, before managing to win her back through outwitting, bloodying and dishonouring his rival.

In music, a song is a composition for voice or voices, performed by singing. A song may be accompanied by musical instruments, or it may be unaccompanied, as in the case of a cappella songs. The lyrics (words) of songs are typically of a poetic, rhyming nature, though they may be religious verses or free prose.

A song may be for a solo singer, a duet, trio, or larger ensemble involving more voices. Songs with more than one voice to a part are considered choral works. Songs can be broadly divided into many different forms, depending on the criteria used. One division is between "art songs", "pop songs", and "folk songs". Other common methods of classification are by purpose (sacred vs secular), by style (dance, ballad, Lied, etc.), or by time of origin (Renaissance, Contemporary, etc.).

A song is a piece of music for accompanied or unaccompanied voice or voices or, "the act or art of singing," but the term is generally not used for large vocal forms including opera and oratorio. However, the term is, "often found in various figurative and transferred sense (e.g. for the lyrical second subject of a sonata...)." The noun "song" has the same etymological root as the verb "to sing" and the Oxford English Dictionary (OED) defines the word to mean "that which is sung" or "a musical composition suggestive of song." The OED also defines the word to mean "a poem" or "the musical phrases uttered by some birds, whales, and insects, typically forming a recognizable and repeated sequence and used chiefly for territorial defence or for attracting mates."