The Jersey Act was a regulation introduced to prevent the registration of most American-bred Thoroughbred horses in the British General Stud Book. It had its roots in the desire of British horse breeders to halt the influx of American-bred racehorses of possibly impure bloodlines during the early 20th century. Many American-bred horses were exported to Europe to race and retire to a breeding career after a number of U.S. states banned gambling, which depressed Thoroughbred racing—and thus breeding—in the United States. The loss of breeding records during the American Civil War and the late beginning of the registration of American Thoroughbreds led many in the British racing establishment to doubt that the American-bred horses were purebred.

In 1913, the Jockey Club and the owners of the General Stud Book passed a regulation named by the foreign press after the Jockey Club's senior steward, Lord Jersey, prohibiting the registration of horses in the book unless all of their ancestors had been registered. Despite protests from American breeders, the regulation remained in force until 1949. Among the factors influencing its relaxation were the racing success of ineligible horses in Europe and the damage caused to British and Irish breeders by the unavailability of French Thoroughbreds during and after the Second World War. In addition, by 1949 the possibly impure ancestors of the American bloodlines had receded far back in most horses' ancestry.

Before the introduction in 1913 of what became popularly known—"with questionable taste" according to a correspondent writing in The Times—as the Jersey Act, Thoroughbred horses in the United Kingdom were registered in the General Stud Book, the stud book for British and Irish Thoroughbreds. The rules allowed a horse to be registered if all of the horse's ancestors were registered in the General Stud Book or if it had been bred outside of Britain or Ireland and was registered in the stud book of its country of origin. Overall the General Stud Book had the most stringent rules for registration of Thoroughbreds at the time, around 1900; other countries, including the United States, France, Australia and Russia, were considered by the British and Irish to be much laxer and to have allowed some non-Thoroughbred horses into their national stud books.

The outlawing of race-track betting in parts of the United States between 1900 and 1913 led to a large influx of American-bred horses into Britain and Ireland, giving rise to fears among British breeders that they would be swamped by the American bloodlines and their own stock would become worthless. The biggest state to outlaw betting was New York, which passed the Hart–Agnew Law in 1908. By 1911, the average price for yearlings sold at auction was at a record low of $230 ($7,760 as of 2024). Before 1900, most horses were imported into Britain to race, and rarely stayed for a breeding career. The outlawing of gambling resulted in large numbers of American horses that could no longer be supported, and many were shipped to Europe for racing. Because of the downturn in the horse market in the United States, it was assumed that most of the horses sent to Europe would stay there permanently and, after retirement from the racetrack, would enter their breeding careers outside of the United States. Between 1908 and 1913, over 1,500 Thoroughbreds were exported from the United States. Those exported included 24 horses who had been or would later become champions – among them Artful, Colin, Henry of Navarre, Peter Pan, and Ballot.

The American Stud Book, the registration book for American Thoroughbreds, was not founded until 1873, much later than the General Stud Book, and the rules for registration required only that a horse have five generations of ancestors in the American Stud Book or other national stud books, unlike the General Stud Book rules. In addition, many breeding records were destroyed during the American Civil War, as fighting during that conflict took place in noted American Thoroughbred breeding centers. The result was that most American Thoroughbreds in 1913 were unable to show an unblemished pedigree according to the General Stud Book rules. Adding to the problem was that American horses were beginning to win the big horse races in England, starting with Iroquois, who won the 1881 Epsom Derby.

J. B. Haggin, an American breeder and owner of Elmendorf Farm, had begun to ship large contingents of horses to England for sale, including the 1908 Grand National steeplechase winner Rubio, and the fear was that if other American breeders followed his lead, the English racing market would be overwhelmed. As a first step, the English racing authorities began to limit the number of training licences at Newmarket Racecourse, turning away a number of American breeders. The General Stud Book rules for registration were also amended in 1909 to restrict registration to horses whose ancestry could be traced entirely to horses already registered in the General Stud Book, but horses registered in other national stud books were still allowed to be imported and registered.

The owners of the General Stud Book, Weatherbys, consulted with the Jockey Club, the United Kingdom's racing authority, and discussions were held about the problems in pedigrees recorded in the American Stud Book. At a meeting of the Jockey Club in spring 1913, Victor Child Villiers, Lord Jersey, the club's senior steward, proposed a resolution limiting the registration of American bloodlines. It passed unanimously in May, and a new regulation was placed in the General Stud Book, Volume 22:

No horse or mare can, after this date, be considered as eligible for admission unless it can be traced without flaw on both sire's and dam's side of its pedigree to horses and mares themselves already accepted in the earlier volumes of the book.