John Diamond (c. 1823 – October 20, 1857), aka Jack or Johnny, was an Irish-American dancer and blackface minstrel performer. Diamond entered show business at age 17 and soon came to the attention of circus promoter P. T. Barnum. In less than a year, Diamond and Barnum had a falling-out, and Diamond left to perform with other blackface performers. Diamond's dance style merged elements of English, Irish, and African dance. For the most part, he performed in blackface and sang popular minstrel tunes or accompanied a singer or instrumentalist. Diamond's movements emphasized lower-body movements and rapid footwork with little movement above the waist.

Diamond was most famous for a series of challenge dances. He regularly advertised that he could defeat all comers in a dancing contest, and he made good on his boasts. However, Diamond soon came to the attention of the dancer whom Barnum had replaced him with, a young black man known as Master Juba. Diamond and Juba fought dance-offs through the mid-1840s; records indicate that Juba won all but one. Accordingly, historian Robert Toll calls Diamond the "greatest white minstrel dancer".

In 1840, Diamond won $500 in a New York City jig competition. Circus proprietor P. T. Barnum took notice and hired the boy, touring him throughout the United States and Europe. Ever the showman, Barnum claimed that the dancer was a mere 12 years old (he was really 17) and billed him as the "King of Diamonds" in advertisements. Diamond performed solo and as part of a pair with blackface singers and musicians. For a time, he was teamed with "the King of Banjo players, and the Emperor of Extravaganza Singers", Billy Whitlock. Their advertisements exclaimed, "Second night of the engagement of the little 'Wirginny Nigger,' only 12 years old, who can outdance the nation, and come some 'Heel and Toe Breakdowns,' that are a caution to all darkies, and no mistake!"

Diamond earned a bad reputation among promoters and managers. One source claimed that his dancing was "considerably better than his temper and disposition." In February 1841, Diamond extorted money from P. T. Barnum and deserted his former mentor to go on a week-long binge of alcohol and women. Barnum sent a letter to his colleagues to caution them against hiring the dancer. Barnum claimed Diamond had "overdrawn the money due him to the amount of $95 and has during the last week expended a hundred dollars in brothels and haunts of dissipation & vice."

Free of Barnum, Diamond played in pairs or groups of three or four blackface performers. In January 1843, he was in a circus with other blackface performers; the program promised "Negro extravaganzas, songs, dances, and locomotive imitations by Whitlock, Diamond, John Daniels and Gardner." The program does not make it clear whether they performed simultaneously or individually. After the Virginia Minstrels formed in 1843, Whitlock convinced Diamond to perform with them in order to increase the group's exposure. In 1845, Diamond was touring the United States with the Old Dominion Circus. In 1849, he was touring with the Olympic Circus. He was still performing regularly in the mid 1850s. Diamond eventually joined the Ethiopian Serenaders minstrel troupe.

In the 1840s, Diamond began a series of challenge dances wherein he dared his rivals to best him in a contest of skill. Diamond publicized his challenges in popular newspapers. Such trials usually had three judges, one who judged time, one style, and one execution. A typical challenge read:

Master Diamond, who delineates the Ethiopian character superior to any other white person, hereby challenges any person in the world to trial of skill at Negro dancing, in all its varieties, for a wager of from $200.00–$1,000.00.

Diamond won match after match in city after city, and his fame grew exponentially. He earned a host of imitators and copycats, many of whom took his name and pretended to be him.