An uncodified constitution is a type of constitution where the fundamental rules often take the form of customs, usage, precedent and a variety of statutes and legal instruments. An explicit understanding of such a constitution can be developed through commentary by the judiciary, government committees or legal experts. In such a constitutional system, all these elements may be (or may not be) recognized by courts, legislators, and the bureaucracy as binding upon government and limiting its powers. Such a framework is sometimes imprecisely called an "unwritten constitution"; however, all the elements of an uncodified constitution are typically written down in a variety of official documents, though not codified in a single document. However, there may be truly "unwritten" constitutional conventions which while not usually legally enforceable may hold just as much sway as the letter of the law.

An uncodified constitution has the advantages of elasticity, adaptability, and resilience. A. V. Dicey described the uncodified constitution as "the most flexible polity in existence." A significant disadvantage, however, is that controversies may arise due to different understandings of the usages and customs that form the fundamental provisions of the constitution.

A new condition or situation of government may be resolved by precedent or passing legislation. Unlike a codified constitution, there are no special procedures for making a constitutional law, and it will not be inherently superior to other legislation. A country with an uncodified constitution lacks a specific moment where the principles of its government were deliberately decided. Instead, these are allowed to evolve according to the political and social forces arising throughout its history.

When viewed as a whole system, the difference between a codified and uncodified constitution is one of degree. Any codified constitution will be overlaid with supplementary legislation and customary practice after a period of time. Conversely, customs and practices that have been observed for long periods in an uncodified manner may be added to the written constitution at various junctures, such as in the case of the two-term limit for presidents of the United States. This custom was observed for nearly a century and a half, unbroken, without any enforcement mechanism until it was ignored by Franklin D. Roosevelt, after which it was added to the written Constitution as mandatory de jure.

The following states can be considered to have uncodified constitutions.

Although there are Constitution Acts, important aspects of the constitutional system are uncodified. The preamble to the Constitution of Canada declares it to be "similar in principle to that of the United Kingdom" (which is uncodified).

This applies at the federal level and to the provinces, although each does have the power to modify or enact their own within their exclusive areas of responsibility. To date only British Columbia has enacted a codified provincial constitution (see Constitution of British Columbia), though the other provinces' roles and powers are spelled out in section 93 of the Constitution Act, 1867, and through amendments to it dealing with particular provinces such as the Manitoba Act and the Newfoundland Act.

Some Chinese academics including legal theorist Jiang Shigong have argued that China has both a written constitution and an unwritten constitution based on the comprehensive leadership of the Communist Party.