Overwhelmed by her clinical depression and the difficulties of caring for her children, she committed suicide just two weeks after publishing her novel, The Bell Jar. Her collection Ariel was published posthumously in 1965. Originally edited and compiled by Hughes, a new version of Ariel was released in 2004 that restores Plath’s original order and the twelve poems missing from the first version. The new version also contains a foreward by her daughter, Frieda Hughes, herself a poet and a painter.

Part of the Confessional movement, alongside her contemporaries Robert Lowell and Anne Sexton, Plath's work in Ariel is intensely personal. The darkly lyric poems address motherhood, sexuality, marriage, and her own experiences with depression. Despite the positive critical reception of her first, more traditional book, Colossus, the poems in Ariel were initially refused by many of the best editors in the country—the New Yorker would not publish more than a few lines of her later work.

Her late poetry, collected in Ariel, includes some of the best-known contemporary poems in the English language, including "Daddy," "Lady Lazarus," "Ariel," and the opening poem of the collection: "Morning Song."