British baseball, also known colloquially in Wales as Welsh baseball, is a bat-and-ball game played in Wales, England, and to a lesser extent in Ireland and Scotland. The game emerged as a distinct sport in Merseyside, Gloucester and South Wales at the end of the 19th century, drawing on the much older game of rounders. Teams in all locations played under the codified rules created by the National Rounders Association (later renamed as the (English Baseball Association), with the game in Wales locally organised first by the South Wales Baseball Association, (still playing under English Baseball Association rules), who in turn were replaced by the Welsh Baseball Union. The Irish Baseball Union were formed in 1933. Both the English Baseball Association and Welsh Baseball Union are members of the International Baseball Board.

In the tradition of bat-and-ball games, British baseball has roots going back centuries, and there are references to "baseball" from the beginning of the eighteenth century, and "rounders" from 1828. Bat-and-ball games in Britain have a long history and a ball and bat game possibly ancestral to rounders and British baseball was attested as early as 1344. A game called "baseball" was attested in 1700 when a vicar in Maidstone decried its playing on a Sunday, and referenced in 1744 in the children's book A Little Pretty Pocket-Book where it was called Base-Ball. Jane Austen also included a passing reference to the game in Northanger Abbey.

David Block and John Thorn, the official historian for Major League Baseball, speculate that this version of baseball may have involved batters hitting with their hand rather than with a bat, akin to modern-day punchball.

The National Rounders Association were based in Merseyside, England, where in April 1886 they were one of a number of locally based sporting institutions with elected representation onto the directorate of the Liverpool Athletic Grounds Company Ltd. Calls were made to follow the success of other working class sports such as football, on Merseyside, and rugby in Wales to adopt a distinct set of rules and bureaucracy. The National Rounders Association were one of a number of national organisations supported by the National Physical Recreation society, created in 1886 with Herbert Gladstone as President, "to promote physical recreation amongst the working classes".

The National Rounders Association duly created a new game, drawing on the much older game of rounders but with new rules that they hoped would appeal to adults. With plans to spread this new game across the entire United Kingdom, Gloucester adopted the sport in 1887, with the Gloucester Rounders Association formed in 1888. In a June 1888 edition of the South Wales Daily News, the now codified rules of this new sport were discussed and it was noted that "the National Rounders Association is the authority". It was also noted that the National Physical Recreation society had supplied a copy of the rules of this new game to the author and "the present mode of playing the game is a great improvement on the old style". In South Wales the game fully took hold in 1889, four clubs were created in South Wales who in 1890 then formed the South Wales Rounders Association. The rules of the new game, created by the National Rounders Association were as follows. "The bases are four, and the runs are made to the right. Eleven is the correct number of players, as at cricket, the bowler, man behind batsmen, longstop, one at each of the bases and cover bases. A regulation bat, not to exceed 30 inches in length or 3 1/2 inches in breadth, is used instead of the 'timmy'. Runs are scored individually to the striker, and collectively to the side; a run for each base that the striker succeeds in reaching without stopping. The ball must be delivered over the striking base above the knee and below the head to be a 'good ball'; two bad balls count one point to the opposite side.

The threat of the American code of baseball in Britain reached a peak between 1889 and 1890, when a national league for that code was formed, the first fully professional baseball league in Britain. The National League of Baseball of Great Britain was created at a meeting at the Criterion, London, alongside the establishment a new baseball association, the new association quickly moved to establish a headquarters at 38 Holborn Viaduct, London. Curiously the National Rounders Association were represented at the meeting. This was the third attempt by American backers to establish their influence over bat and ball games in Britain, in the 1870s, baseball teams from the United States, including the Boston Red Stockings and Philadelphia Athletics, toured the United Kingdom but with limited success. In 1889 the wealthy Albert Goodwill Spalding used his position as a former star player of the American code, and as a leading sporting goods supplier, to arrange another tour of the United Kingdom by American baseball stars including the Chicago White Stockings.

In 1892, in “finding there was so much prejudice against the name” the National Rounders Association and the South Wales Rounders Association dropped ‘rounders’ and replaced this with ‘baseball’. This resulted in the governing body, the National Rounders Association, being known as the English Baseball Association and the South Wales Rounders Association becoming the South Wales Baseball Association, who at their annual meeting in Newport, in 1893, "decided to continue to play under English rules". By the end of the 1892 season, baseball teams from Liverpool and Lancashire were invited to play matches at Cardiff Arms Park with the express purpose of popularising "the improved version of the old-fashioned game of rounders". In May 1892, it was reported that the Gloucestershire Rounders Association had become the Gloucester English Baseball Association, with leagues already under way. By June 1892 baseball in Gloucestershire was in full swing, but ultimately this was a short lived enterprise.

In June 1892 Newton Crane, the President of the National Baseball Association (the body responsible for the overseeing of the American rules of baseball in Great Britain) wrote an open letter in the Liverpool Mercury, stating that Derby Baseball Club had accepted the challenge of the Chief Executive Officer of the National Rounders Association (by now renamed as the English Baseball Association), that a team of the best British baseball players in the country could defeat a team of the best ‘American rules’ baseball players in Britain. The matches would “be played according to the genuine or regular baseball rules”, with a match each in Liverpool and Derby. By 1894 the Liverpool and District Baseball League was established and the game was flourishing on Merseyside and parts of Lancashire and South Wales. In April 1899, at the annual meeting of the English Baseball Association in Liverpool, it was resolved that the rules of the game would be changed, to speed up play and tackle the “waiting tactics adopted by batsmen”. The change would see batters face two “good balls” instead of three, and that on the third “good ball” the batter would get an “extra” ball.