Presumed knowledge of the law is the principle in jurisprudence that one is bound by a law even if one does not know of it. It has also been defined as the "prohibition of ignorance of the law".

The concept comes from Roman law, and is expressed in the brocard ignorantia legis non excusat (ignorance of the law is no excuse).

The essential public character of a law requires that the law must apply to anyone in the jurisdiction where the law applies. Thus, no one can justify his conduct on the grounds that he was not aware of the law.

Generally, a convention exists (by some called "the essential preliminary rule") by which the laws are issued and rendered accessibile by methods, authors and means that are simple and well known: the law is readable in certain places (some systems prescribe that a collection of the laws is copied in every local city council), is made by certain authorities (usually sovereign, government, parliament, and derivative bodies), and enters into effect in certain ways (many systems for instance prescribe a certain number of days - often 15 - after issue). This is commonly intended as a constitutional regulation, and in fact many constitutions or statutes exactly describe the correct procedures.

However, some recent interpretations weaken this concept. Particularly in civil law, regard can be had to the difficulty of being informed of the existence of a law considering the lifestyle of the average citizen. On the penal side, the quality of the knowledge of the law can affect the evaluation of the animus nocendi or the mens rea, in that certain subjective conditions can weaken personal responsibility.

The theme was widely discussed, also for political reasons, at the time of the Enlightenment and in the 18th century, given the heavy proportion of illiterate citizens in European countries (who would have some difficulties being aware of all the laws in a country). It was then argued that both the presumed knowledge and the heavily increasing corpus of national legislation were working in favour of lawyers rather than citizens.

In recent times, some authors have considered this concept as an extension of (or at least as analogous to) the other ancient concept (typical of criminal law) that no one can be punished under a law that was issued after the action was committed (non-retroactivity of the law. See ex post facto). This interpretation is however disputed, given that the matter would hierarchically more properly refer to a constitutional doctrine rather than to a civil or penal one.