A by-law (bye-law, by(e)law, by(e) law), is a set of rules or law established by an organization or community so as to regulate itself, as allowed or provided for by some higher authority. The higher authority, generally a legislature or some other government body, establishes the degree of control that the by-laws may exercise. By-laws may be established by entities such as a business corporation, a neighbourhood association, or depending on the jurisdiction, a municipality.

In the United Kingdom and some Commonwealth countries, the local laws established by municipalities are referred to as by(e)-laws because their scope is regulated by the central governments of those nations. Accordingly, a bylaw enforcement officer is the Canadian equivalent of the American Code Enforcement Officer or Municipal Regulations Enforcement Officer. In the United States, the federal government and most state governments have no direct ability to regulate the single provisions of municipal law. As a result, terms such as code, ordinance, or regulation, if not simply law, are more common.

The Merriam-Webster Dictionary indicates that the origin of the word by-law is from the English word bilawe, probably from Old Norse *bȳlǫg, from Old Norse bȳr town + lag-, lǫg law. The earliest use of the term, which originates from the Viking town law in the Danelaw, wherein by is the Old Norse word for a larger settlement as in Whitby and Derby (compare with the modern Danish-Norwegian word by meaning town, or the modern Swedish word by, meaning village). However, it is also possible that this usage was forgotten and the word was "reinvented" in modern times through the use of the adverbial prefix by- giving the meaning of subsidiary law or side-law (as in byway). In any case, it is incorrect to claim that the word is related to the prepositional phrase "by law"; that is a modern conjecture contradicted by the evidence.

Municipal by-laws are public regulatory laws; which apply in a certain area. The main difference between a by-law and a law passed by a national/federal or regional/state body is that a by-law is made by a non-sovereign body, which derives its authority from another governing body, and can only be made on a limited range of matters. A local council or municipal government derives its power to pass laws through a law of the national or regional government which specifies what things the town or city may regulate through by-laws. It is therefore a form of delegated legislation. Within its jurisdiction and specific to those areas mandated by the higher body, a municipal by-law is no different from any other law of the land, and can be enforced with penalties, challenged in court, and must comply with other laws of the land, such as the country's constitution. Municipal by-laws are often enforceable through the public justice system, and offenders can be charged with a criminal offence for breach of a by-law. Common by-laws include vehicle parking and stopping regulations, animal control, building and construction, licensing, noise, zoning and business regulation, and management of public recreation areas.

Under Article 94 of the Constitution of Japan, regional governments have limited autonomy and legislative powers to create by-laws. In practice, such powers are exercised in accordance with the Local Autonomy Law.

By-laws therefore constitute part of the legal system subordinate to the Japanese constitution. In terms of its mandatory powers and effective, it is considered the lowest of all legislation possible.

Such powers are used to govern the following:

In the United Kingdom, by-laws are laws of local or limited application made by local councils or other bodies, using powers granted by an Act of Parliament, and so are a form of delegated legislation.