The Marriage of Lunatics Act 1811 was an act of Parliament of the United Kingdom implemented under the reign of George III of Great Britain.

It was intended "to prevent the marriage of Lunatics" and make all marriages to Lunatics prior to and after the bill, whether diagnosed before marriage or otherwise, "null and void to all intents and purposes whatsoever".

[2] The act reads as follows: The act was fully repealed in 2021 on the commencement of the Assisted Decision-Making (Capacity) Act 2015 in the Republic of Ireland.

[3][4][5] It had earlier been repealed in the United Kingdom by the Mental Health Act 1959 and the Mental Health (Scotland) Act 1960.