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The Ash Grove (Welsh: Llwyn Onn) is a traditional Welsh folk song whose melody has been set to numerous sets of lyrics. The best-known version was written in English by Thomas Oliphant in the 19th century.

The first published version of the tune was in 1802 in The Bardic Museum, a book written by the harpist Edward Jones. About four years later a version with words appeared, under the name Llwyn Onn. It tells of a sailor's love for "Gwen of Llwyn". At the end of the song, Gwen dies, and in one version of the piece, the writer talks about him mourning and that she is lying " 'neath the shades of the lonely ash grove". The tune might be much older, as a similar air appears in The Beggar's Opera by John Gay (1728), in the song Cease Your Funning. In 1922, however, the English folksong collector Frank Kidson claimed that Gay's air derives from the morris dance tune Constant Billy, which is first known in Playford's Dancing Master (1665).

The first known English-language version of "The Ash Grove" was published in 1862, in Volume I of Welsh Melodies, with Welsh and English Poetry, compiled by the harpist John Thomas, with Welsh words by John Jones (Talhaiarn) and English words by Thomas Oliphant. The first verse of this version is incorporated into a different interpretation by the English dramatist and translator John Oxenford.