The National Association of Base Ball Players (NABBP) was the first organization governing American baseball (spelled as two words in the 19th century).

The first convention of 16 New York City area clubs was held at Smith's Hotel, 462 Broome Street in January 1857. One delegate from each club had previously been assigned to a committee, whose task was to draw up a set of regulations governing the game. Their report was adopted at the final meeting of the convention on February 25, effectively bringing to an end the Knickerbocker era, when games were played under rules largely at the discretion of individual clubs. Besides governing the playing rules and its own organization, the Association established standards for official scoring (reporting), "match" play, a championship, amateurism, and the integrity of the contest (then known as hippodroming). Following trends in the evolution of the sport, in the 1869 season it accepted professionalism in its ranks.

The last convention, held in 1871, with hundreds of members represented only via state associations, provoked the establishment of separate professional and amateur associations. The succeeding National Association of Professional Base Ball Players is considered the first professional sports league; through 1875 it governed professional baseball and practically set playing rules for all. Because the amateur successor never attracted many members and it convened only a few times, the NABBP is sometimes called "the amateur Association" in contrast to its professional successor.

Before the American Civil War (1861–1865), the beginnings of baseball competed for public interest with cricket and regional variants of baseball, notably town ball played in Philadelphia and the Massachusetts Game played in New England. In the 1860s, aided by the War, "New York" style baseball (as often played by Union Army troops passing the uneventful days of camp life between occasional skirmishes and battles) expanded through various corps, regiments, brigades, and other units into a national game. In addition, the parallel civilian NABBP, as its governing body, expanded into a true national organization, although most of the strongest clubs remained those based in New York City, Brooklyn and Philadelphia. By the end of 1865, almost 100 clubs were members of the "National Association" organization. By 1867, it had over 400 members, including some clubs from as far away as the Pacific Ocean west coast in San Francisco and south to the Gulf of Mexico coast in Louisiana. Because of this growth, regional and state organizations began to assume a more prominent role in the governance of the sport. Baseball's exploding popularity, however, was not confined to the NABBP organization, whose core lay in the vicinity of New York City; there were thousands of organized baseball clubs nationwide by 1870, the majority of which were not Association members. For example, on the eve of the Civil War there were no fewer than seven baseball teams in distant New Orleans, none of which belonged to the NABBP; a tabulation by historian Richard Herschberger turned up over 900 baseball clubs by 1860, in which year the Association's membership stood at 60.

The NABBP was initially established upon principles of amateurism. However, even early in its history some star players, such as James Creighton of the Excelsior club of Brooklyn, received compensation, either secretly or through emoluments. In 1866, the NABBP investigated the Athletic club of Philadelphia for paying three players, including Lip Pike, but took no action against either the club or the players. However, as inter-club competition became more intense and clubs sought to secure the services of the best players, unofficial payments became common. To maintain the integrity of the game, at its December 1868 meeting the NABBP established a professional category for the coming 1869 season, and clubs who desired to pay players were now free to do so without sanction.

Cincinnati's Red Stockings was the first to so declare and was among the most aggressive in recruiting the best available players (as a result of which they became the most dominant club of the era). Twelve clubs, including most of the strongest in the NABBP, ultimately declared themselves professional for the 1869 season.

Conflict arose, however, between amateur and professional interests. Important issues included how the championship was to be decided and regulating players jumping from one team to another. As a result, after three years of this experiment, most of the leading professional clubs broke away in 1871 to found the separate National Association of Professional Base Ball Players (NAPBBP). The earlier NABBP continued for about two more years with a more diminished status before disbanding into state and regional aggregations.

Contrary to the organization name, NABBP members were clubs, not individual players. Generally the clubs joined the association and retained membership by sending delegates to the annual convention, usually in the December preceding each season (the ancestor of modern Major League Baseball's so-called Winter Meetings). Membership mediated by state associations was introduced only after ten years; then dozens of clubs from a distant state could join and remain in the NABBP by organizing a state association whose delegates would participate in the national meeting.