A Fleet Marriage was a clandestine marriage arranged in London's Fleet Prison during the 17th, 18th and 19th centuries.

By the law of England a marriage was recognized as valid, so long as the ceremony was conducted by a person in holy orders, even if those orders were not of the Church of England. Neither banns nor licence were necessary, and the time and place were alike immaterial. Out of this state of the marriage law, in the period of laxness which succeeded the Commonwealth, resulted innumerable clandestine marriages. They were contracted at first to avoid the expenses attendant on the public ceremony, but an act of 1696, which imposed a penalty of 100 pounds on any clergyman who celebrated, or permitted another to celebrate, a marriage otherwise than by banns or licence, acted as a considerable check. To clergymen imprisoned for debt in Fleet Prison, however, such a penalty had no terrors, for they had neither liberty, money nor credit to lose by any proceedings the bishop might institute against them.

The earliest recorded date of a Fleet Marriage is 1613, while the earliest recorded in a Fleet Register took place in 1674, but it was only on the prohibition of marriage without banns or licence that they began to be clandestine. Then arose keen competition, and many of the Fleet parsons and tavern-keepers in the neighborhood fitted up a room in their respective lodgings or houses as a chapel, and employed touts to solicit custom for them. The scandal and abuses brought about by these clandestine marriages became so great that they became the object of special legislation.

In 1753 Lord Hardwicke's Marriage Act was passed, which required, under pain of nullity, that banns should be published according to the rubric, or a licence obtained, and that, in either case, the marriage should be solemnized in church; and that in the case of minors, marriage by licence must be by the consent of parent or guardian. This act had the effect of putting a stop to these clandestine marriages, so far as England was concerned, and henceforth couples had to fare to Gretna Green. The Fleet Registers, consisting of about two or three hundred large registers and about a thousand rough or pocket books, eventually came into private hands, but were purchased by the government in 1821, and are now deposited in the office of the registrar-general, Somerset House. Their dates range from 1686 to 1754. In 1840 they were declared not admissible as evidence to prove a marriage.