The Royal Marriages Act 1772 (12 Geo. 3. c. 11) was an act of the Parliament of Great Britain which prescribed the conditions under which members of the British royal family could contract a valid marriage, in order to guard against marriages that could diminish the status of the royal house. The right of veto vested in the sovereign by this Act provoked severe adverse criticism at the time of its passage.

The whole act was repealed as a result of the 2011 Perth Agreement, which came into force on 26 March 2015. Under the Succession to the Crown Act 2013, the first six people in the line of succession need permission to marry if they and their descendants are to remain in the line of succession.

The act said that no descendant of King George II, male or female, other than the issue of princesses who had married or might thereafter marry "into foreign families", could marry without the consent of the reigning monarch, "signified under the great seal and declared in council". That consent was to be set out in the licence and in the register of the marriage, and entered in the books of the Privy Council. Any marriage contracted without the consent of the monarch was to be null and void.

However, any member of the royal family over the age of 25 who had been refused the sovereign's consent could marry one year after giving notice to the Privy Council of an intention to marry, unless both houses of Parliament expressly declared their disapproval. There was, however, no instance in which the sovereign's consent in Council was formally refused, though there was one where it was sought but the request ignored and others where it was not sought because it was likely to be refused.

The act further made it a crime to perform or participate in an illegal marriage of any member of the royal family. This provision was repealed by the Criminal Law Act 1967.

The act was proposed by George III as a direct result of the marriage in 1771 of his brother, Prince Henry, Duke of Cumberland and Strathearn, to the commoner Anne Horton, widow of Christopher Horton and daughter of the first Lord Irnham, MP. Royal assent was given to the Act on 1 April 1772, and it was only on 13 September following that the king learned that another brother, Prince William Henry, Duke of Gloucester and Edinburgh, had in 1766 secretly married Maria, the illegitimate daughter of Sir Edward Walpole and the widow of the 2nd Earl Waldegrave. Both alliances were considered highly unsuitable by the king, who "saw himself as having been forced to marry for purely dynastic reasons".

The Act rendered void any marriage wherever contracted or solemnised in contravention of it. A member of the royal family who contracted a marriage that violated the Act did not thereby lose his or her place in the line of succession, but the offspring of such a union were made illegitimate by the voiding of the marriage and thus lost any right to succeed.

The Act applied to Catholics, even though they are ineligible to succeed to the throne. It did not apply to descendants of Sophia of Hanover who are not also descendants of George II, even though they are still eligible to succeed to the throne.