The M'Naghten rule(s) (pronounced, and sometimes spelled, McNaughton) is a legal test defining the defence of insanity, first formulated by House of Lords in 1843. It is the established standard in UK criminal law, and versions have also been adopted in some US states (currently or formerly), and other jurisdictions, either as case law or by statute. Its original wording is a proposed jury instruction:that every man is to be presumed to be sane, and ... that to establish a defence on the ground of insanity, it must be clearly proved that, at the time of the committing of the act, the party accused was labouring under such a defect of reason, from disease of the mind, as not to know the nature and quality of the act he was doing; or if he did know it, that he did not know he was doing what was wrong.

Daniel M'Naghten was a Scottish woodturner who assassinated English civil servant Edward Drummond while suffering from paranoid delusions. Following his trial and its aftermath, his name has been given to the legal test of criminal insanity in England and other common law jurisdictions known as the M'Naghten rules.