Public law is the part of law that governs relations between legal persons and a government, between different institutions within a state, between different branches of governments, as well as relationships between persons that are of direct concern to society. Public law comprises constitutional law, administrative law, tax law and criminal law, as well as all procedural law. Laws concerning relationships between individuals belong to private law.

The relationships public law governs are asymmetric and unequalized. Government bodies (central or local) can make decisions about the rights of persons. However, as a consequence of the rule-of-law doctrine, authorities may only act within the law (secundum et intra legem). The government must obey the law. For example, a citizen unhappy with a decision of an administrative authority can ask a court for judicial review.

The distinction between public law and private law dates back to Roman law, where the Roman jurist Ulpian (c. 170 – 228) first noted it. It was later adopted to understand the legal systems both of countries that adhere to the civil-law tradition, and of those that adhere to common-law tradition.

The borderline between public law and private law is not always clear. Law as a whole cannot neatly be divided into "law for the State" and "law for everyone else". As such, the distinction between public and private law is largely functional rather than factual, classifying laws according to which domain the activities, participants, and principal concerns involved best fit into. This has given rise to attempts to establish a theoretical understanding for the basis of public law.