Eckford of Brooklyn, or simply Eckford, was an American baseball club from 1855 to 1872. When the Union Grounds opened on May 15, 1862 for baseball in Williamsburg, Brooklyn, it became the first enclosed baseball grounds in America. Three clubs called the field on the corner of Marcy Avenue and Rutledge Street home; however, the Eckford of Brooklyn were the most famous tenant. They played more games than any other club that year (7) and won the "national" championship, repeating the feat in 1863. During that two year period, the Eckfords won 22 straight matches which was the longest undefeated and untied streak to date. In the late 1860s, they were one of the pioneering professional clubs, although probably second to Mutual of New York at the home park. In its final season, Eckford entered the second championship of the National Association, the first professional baseball league in America, so it is considered a major league club by those who count the NA as a major league.

Union Grounds was a baseball park located in the Williamsburg section of Brooklyn, New York. The grounds opened in 1862, its inaugural match being played on May 15. It was the first baseball park enclosed entirely by a fence, thereby allowing proprietor William Cammeyer or his tenant to charge admission. This permitted paying customers to watch the games from benches in a stand while non-paying spectators could only watch from embankments outside the grounds.