The Act of Union 1536 combined England and Wales into a single state. It was passed during the reign of King Henry VIII of England. His father, Henry VII was Welsh-born and very conscious of it, and Henry VIII declared himself proud of his Welsh blood. His motive in merging the two countries was not mere domination; Wales benefited in many ways from the legislation.

Although the native Welsh rulers had been subdued by the Statute of Rhuddlan in 1284, the English king remained concerned about the power of the Marcher lords and his government's general lack of control over the principality. He therefore instructed his chief administrator, Thomas Cromwell, to seek a solution. The effect of the act was to make Wales an integral part of the Kingdom of England:

The country of Wales justly and righteously is … incorporated, annexed, united and subject to and under the imperial Crown of the Realm, as a very member and joint of the same.

It was not unpopular with the Welsh, who recognised that it would help give them equality with their neighbours in law. Under the act, the Marcher lordships were abolished and replaced by counties. For the first time, Wales was entitled to send members to the parliament at Westminster. Justices of the Peace were created to administer the law and justice at a local level, in line with the English practice.

Another effect of the act was to outlaw the Welsh language from official use, replacing it with English. This did not trouble the landed gentry, who were already largely anglicised, but it made life difficult for the common people, who were no longer able, for example, to understand court proceedings.

The last parts of the Act relating to language, were repealed in 1993 by the Welsh Language Act 1993 .