In the common law legal system, a plea of autrefois acquit (French for 'previously acquitted') means the defendant claims to have been previously acquitted of the same offence, on substantially the same evidence, and that hence he or she cannot be tried again. A plea of autrefois acquit can be combined with a plea of not guilty. This plea is one of the peremptory pleas. It is a term of Law French.
Related doctines include res judicata and, in the criminal context, a plea in bar of double jeopardy.