In Norse mythology, Ragnarök (also Ragnarok; /ˈræɡnərɒk/ ^ⓘ RAG-nə-rok or /ˈrɑːɡ-/ RAHG-; Old Norse: Ragnarǫk [ˈrɑɣnɑˌrɒk]) is a foretold series of impending events, including a great battle in which numerous great Norse mythological figures will perish (including the gods Odin, Thor, Týr, Freyr, Heimdall, and Loki); it will entail a catastrophic series of natural disasters, including the burning of the world, and culminate in the submersion of the world underwater. After these events, the world will rise again, cleansed and fertile, the surviving and returning gods will meet, and the world will be repopulated by two human survivors, Líf and Lífþrasir. Ragnarök is an important event in Norse mythology and has been the subject of scholarly discourse and theory in the history of Germanic studies.

The event is attested primarily in the Poetic Edda, compiled in the 13th century from earlier traditional sources, and the Prose Edda, written in the 13th century by Snorri Sturluson. In the Prose Edda and in a single poem in the Poetic Edda, the event is referred to as Ragnarøkkr (Old Norse for 'Twilight of the Gods'), a usage popularised by 19th-century composer Richard Wagner with the title of the last of his Der Ring des Nibelungen operas, Götterdämmerung (1876), which is "Twilight of the Gods" in German.

The Old Norse compound word ragnarok has a long history of interpretation. Its first element is clear: ragna, the genitive plural of regin (n. pl.) 'the ruling powers, gods.' The second element is more problematic, as it occurs in two variants, -rök and -røkkr. Writing in the early 20th century, philologist Geir T. Zoëga treats the two forms as two separate compounds, glossing ragnarök as 'the doom or destruction of the gods' and ragnarøkkr as 'the twilight of the gods.' The plural noun rök has several meanings, including 'development', 'origin', 'cause', 'relation', 'fate.' The word ragnarök as a whole is then usually interpreted as the 'final destiny of the gods.'

The singular form ragnarøk(k)r is found in a stanza of the Poetic Edda poem Lokasenna, and in the Prose Edda. The noun røk(k)r means 'twilight' (from the verb røkkva 'to grow dark'), suggesting a translation 'twilight of the gods.' This reading was widely considered a result of folk etymology, or a learned reinterpretation of the original term due to the merger of /ɔ/ (spelled ǫ) and /ø/ (spelled ø) in Old Icelandic after c. 1200 (nevertheless giving rise to the calque Götterdämmerung 'Twilight of the Gods' in the German reception of Norse mythology).

Other terms used to refer to the events surrounding Ragnarök in the Poetic Edda include aldar rök (aldar means age, 'end of an age') from a stanza of Vafþrúðnismál, tíva rök from two stanzas of Vafþrúðnismál, þá er regin deyja ('when the gods die') from Vafþrúðnismál, unz um rjúfask regin ('when the gods will be destroyed') from Vafþrúðnismál, Lokasenna, and Sigrdrífumál, aldar rof ('destruction of the age') from Helgakviða Hundingsbana II, regin þrjóta ('end of the gods') from Hyndluljóð, and, in the Prose Edda, þá er Muspellz-synir herja ('when the sons of Muspell move into battle') can be found in chapters 18 and 36 of Gylfaginning.

The Poetic Edda contains various references to Ragnarök:

In the Poetic Edda poem Völuspá, references to Ragnarök begin from stanza 40 until 58, with the rest of the poem describing the aftermath. In the poem, a völva (a female seer) recites information to Odin. In stanza 41, the völva says:

Fylliz fiǫrvi   feigra manna,
rýðr ragna siǫt   rauðom dreyra.
Svǫrt verða sólskin   of sumor eptir,
veðr ǫll válynd.   Vitoð ér enn, eða hvat?