Ignorantia juris non excusat or Ignorantia legis neminem excusat (Latin for "ignorance of the law is no excuse") is a legal doctrine holding that a person who is unaware of a law may not escape punishment for violating the law merely because they were unaware of the law; that is, persons have presumed knowledge of the law. The rationale behind the doctrine is that if ignorance was an excuse, persons charged with criminal offenses would merely claim they were unaware of the law in question to escape punishment. As such, from the standpoint of the law, all persons are aware of all laws, even though from a practical standpoint this would be impossible, even for someone with substantial legal training. However, it is well settled that persons engaged in undertakings outside what is common for a normal person, such as running a nuclear power plant, will make themselves aware of the laws necessary to engage in that undertaking.

The doctrine assumes that the law in question has been properly promulgated, for example, by being printed in a government gazette.

However, although ignorance may not go to guilt, it can be a consideration in sentence, particularly where the law is unclear or the defendant sought advice from law enforcement or regulatory officials. For example, in one Canadian case, a person was charged with being in possession of gambling devices after they had been advised by customs officials that such devices were legal to import into Canada. Although the defendant was found guilty of the crime, in the circumstances they were granted an absolute discharge .

In addition, there were, particularly in the days before satellite communication and cellular phones, persons who could genuinely be ignorant of the law due to distance or isolation. For example, in a case in British Columbia, a pair of hunters were acquitted of game offenses where the law was changed during the period of time they were in the wilderness hunting.