In criminal law, automatism is a rarely used criminal defence. It is one of the mental condition defences that relate to the mental state of the defendant. Automatism can be seen variously as lack of voluntariness, lack of culpability (unconsciousness) or excuse. Automatism means that the defendant was not aware of his or her actions when making the particular movements that constituted the illegal act.

For example, Esther Griggs in 1858 threw her child out of a first floor window believing that the house was on fire, while having a sleep terror. In 2002, Peter Buck, lead guitarist of the band R.E.M., was cleared of several charges, including assault, which resulted from automatism brought on by a bad interaction between alcohol and sleeping pills. In a 2009 case in Aberporth in west Wales, Brian Thomas strangled his wife in their camper van, also during a sleep terror, when he mistook his wife for an intruder. The defence of automatism is denying that the person was acting in the sense that the criminal law demands. As such it is really a denial-of-proof – the defendant is asserting that the offence is not made out. The prosecution does not have to disprove the defence as is sometimes erroneously reported; the prosecution has to prove all the elements of the offence including the voluntary act requirement. Automatism is a defence even against strict liability crimes like dangerous driving, where no intent is necessary.

There are several limitations to the defence of automatism in English law. Prior fault generally excludes automatism. Intoxication generally excludes automatism, even when involuntary. Any defence that rests on insanity comes under the M'Naghten rules. Under English law internal causes of automatism are generally judged to be insane automatism and so result in the special verdict (not guilty by reason of insanity) rather than simple acquittal.