Non est factum (Latin for "it is not [my] deed") is a defence in contract law that allows a signing party to escape performance of an agreement "which is fundamentally different from what he or she intended to execute or sign". A claim of non est factum means that the signature on the contract was signed by mistake, without knowledge of its meaning. A successful plea would make the contract void ab initio.

According to Saunders v Anglia Building Society [1971] AC 1004, applied in Petelin v Cullen [1975], the strict requirements necessary for a successful plea are generally that:

1. The person pleading non est factum must belong to "class of persons, who through no fault of their own, are unable to have any understanding of the purpose of the particular document because of blindness, illiteracy or some other disability". The disability must be one requiring the reliance on others for advice as to what they are signing.
2. The "signatory must have made a fundamental mistake as to the nature of the contents of the document being signed", including its practical effects.
3. The document must have been radically different from one intended to be signed.

Non est factum is difficult to claim as it does not allow for negligence on the part of the signatory; i.e. failure to read a contract before signing it, or carelessness,^{: para 12} will not allow for non est factum. Furthermore, the Court has noted that there is a heavy onus that must be discharged to establish this defence as it is an "exceptional defence".^: 359–60