Ignorantia juris non excusat is a Latin maxim meaning ignorance of law is not an excuse to a criminal charge. The maxim, ignorantia juris non excusat is applicable to civil as well as criminal jurisprudence in the U.S. It was also recognized in courts of chancery as well as at common law. Ignorantia juris non excusat is everywhere recognized and uniformly enforced. In People v. Klock, 55 Misc. 46 (N.Y. County Ct. 1907), it was held that ignorance of a municipal law is not allowed to excuse anyone who is of the age of discretion from the penalty for the breach of it, for every such person is bound to know the law of the land regulating his conduct, and s/he is presumed so to know.
The purpose of this maxim is that if ignorance is considered an excuse, a person charged with criminal offenses or a subject of a civil lawsuit would merely claim that s/he is unaware of the law in question to avoid liability. Ignorantia juris non excusat is also known as ignorantia legis non excusat.