Theatre or theater: which is correct?

Correct
theatre (UK) / theater (US; theatre is also used in US names)
Avoid
theater (in UK English)
Summary

Correct: theatre (UK) / theater (US; theatre is also used in US names) — Avoid: theater (in UK English). Theatre / theater = a building where plays are performed or films shown; the writing and production of plays.

Both spellings are correct in their own variety; pick one and use it consistently.

Part of speech
noun
Source
Oxford / Merriam-Webster

Why

Oxford: theatre is the only British spelling; Merriam-Webster gives theater as the American standard but records theatre as a variant that many American venues and companies use in their names. Follow the house style, and keep a proper name's own spelling.

Examples

  • We saw the play at the National Theatre (UK) / a local theater (US).
  • She studied theatre at university.
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Checked against
Merriam-Webster / Oxford
Spelling authority
Merriam-Webster · Oxford English Dictionary / Oxford spelling · SCOWL
Commonly confused and non-standard forms
64
Dictionary
Hunspell en_US / en_GB / en_AU / en_CA · Dictionary license: MIT AND BSD (SCOWL combined licence)