"Ode to a Nightingale" is a poem by John Keats, one of his 1819 odes. It was written either in the garden of the Spaniards Inn, Hampstead, London, or, according to Keats' friend Charles Armitage Brown, under a plum tree in the garden of Keats' house at Wentworth Place, also in Hampstead. According to Brown, a nightingale had built its nest near the house that he shared with Keats in the spring of 1819. Inspired by the bird's song, Keats composed the poem in one day. It was first published in Annals of the Fine Arts the following July. The poem is one of the most frequently anthologized in the English language.

"Ode to a Nightingale" is a personal poem which describes Keats' journey into the state of negative capability. The tone rejects the optimistic pursuit of pleasure found within Keats's earlier poems and, instead, explores the themes of nature, transience and mortality, the latter being particularly relevant to Keats. The nightingale described experiences a type of death but does not actually die. Instead, it is capable of living through its song, a fate that humans cannot expect. The poem ends with an acceptance that pleasure cannot last and that death is an inevitable part of life, as Keats imagines the loss of the physical world and sees himself dead—a "sod" over which the nightingale sings.

Many critics favor "Ode to a Nightingale" for its themes but some believe that it is structurally flawed because the poem sometimes strays from its main idea.

Of Keats's six major odes of 1819, "Ode to Psyche", was probably written first and "To Autumn" written last. Some time between these two, he wrote "Ode to a Nightingale". It is possible that "Ode to a Nightingale" was written between 26 April and 18 May 1819, based on weather conditions and similarities between images in the poem and those in a letter sent to Fanny Brawne on May Day. The poem was composed at the Hampstead house Keats shared with his friend Charles Armitage Brown, possibly while sitting beneath a plum tree in the garden. According to Brown, Keats finished the ode in just one morning: "In the spring of 1819 a nightingale had built her nest near my house. Keats felt a tranquil and continual joy in her song; and one morning he took his chair from the breakfast-table to the grass-plot under a plum-tree, where he sat for two or three hours. When he came into the house, I perceived he had some scraps of paper in his hand, and these he was quietly thrusting behind the books. On inquiry, I found those scraps, four or five in number, contained his poetic feelings on the song of the nightingale." Brown's account is personal, as he claimed the poem was directly influenced by his house and preserved by his own doing. However, Keats relied on both his own imagination and other literature as sources for his depiction of the nightingale.

The exact date of "Ode to a Nightingale", as well as those of "Ode on Indolence", "Ode on Melancholy", and "Ode on a Grecian Urn", is unknown, as Keats dated all as 'May 1819'. However, he worked on the four poems together, and there is a unity in both their stanza forms and their themes. The exact order in which the poems were written is also unknown, but they form a sequence within their structures. While Keats was writing "Ode on a Grecian Urn" and the other poems, Brown transcribed copies of the poems and submitted them to Richard Woodhouse. During this time, Benjamin Haydon, Keats' friend, was given a copy of "Ode to a Nightingale", and he shared the poem with the editor of the Annals of the Fine Arts, James Elmes. Elmes paid Keats a small sum of money, and the poem was published in the July issue. The poem was later included in Keats' 1820 collection of poems Lamia, Isabella, The Eve of St Agnes, and Other Poems.

"Ode to a Nightingale" was probably the first of the middle set of four odes that Keats wrote following "Ode to Psyche", according to Brown. This is further evidenced by the poems' structures. Keats experimentally combines two different types of lyrical poetry: the odal hymn and the lyric of questioning voice that responds to the odal hymn. This combination of structures is similar to that in "Ode on a Grecian Urn". In both poems, the dual form creates a dramatic element within the text. The stanza form of the poem is a combination of elements from Petrarchan sonnets and Shakespearean sonnets.

Keats incorporates a pattern of alternating historically "short" and "long" vowel sounds in his ode. In particular, line 18 ("And purple-stained mouth") has the historical pattern of "short" followed by "long" followed by "short" and followed by "long". This alternation is continued in longer lines, including line 31 ("Away! away! for I will fly to thee") which contains five pairs of alternations. However, other lines, such as line 3 ("Or emptied some dull opiate to the drains") rely on a pattern of five "short" vowels followed by "long" and "short" vowel pairings until they end with a "long" vowel. These are not the only combination patterns present, and there are patterns of two "short" vowels followed by a "long" vowel in other lines, including 12, 22, and 59, which are repeated twice and then followed up with two sets of "short" and then "long" vowel pairs. This reliance on vowel sounds is not unique to this ode, but is common to Keats's other 1819 odes and his Eve of St. Agnes.

The poem incorporates a complex reliance on assonance—the repetition of vowel sounds—in a conscious pattern, as found in many of his poems. Such a reliance on assonance is found in very few English poems. Within "Ode to a Nightingale", an example of this pattern can be found in line 35 ("Already with thee! tender is the night"), where the "ea" of "Already" connects with the "e" of "tender" and the "i" of "with" connects with the "i" of "is". This same pattern is found again in line 41 ("I cannot see what flowers are at my feet") with the "a" of "cannot" linking with the "a" of "at" and the "ee" of "see" linking with the "ee" of "feet". This system of assonance can be found in approximately a tenth of the lines of Keats's later poetry.