National Basketball Association (NBA), professional basketball league formed in the United States in 1949 by the merger of two rival organizations, the National Basketball League (founded 1937) and the Basketball Association of America (founded 1946).

The Basketball Association of America was founded in 1946 by owners of the major ice hockey arenas in the Northeastern and Midwestern United States and Canada. On November 1, 1946, in Toronto, Ontario, Canada, the Toronto Huskies hosted the New York Knickerbockers at Maple Leaf Gardens, in a game the NBA now refers to as the first game played in NBA history.The first basket was made by Ossie Schectman of the Knickerbockers. Although there had been earlier attempts at professional basketball leagues, including the American Basketball League and the NBL, the BAA was the first league to attempt to play primarily in large arenas in major cities. During its early years, the quality of play in the BAA was not significantly better than in competing leagues or among leading independent clubs such as the Harlem Globetrotters. For instance, the 1948 ABL finalist Baltimore Bullets moved to the BAA and won that league's 1948 title, and the 1948 NBL champion Minneapolis Lakers won the 1949 BAA title. Prior to the 1948–49 season, however, NBL teams from Fort Wayne, Indianapolis, Minneapolis, and Rochester jumped to the BAA, which established the BAA as the league of choice for collegians looking to turn professional.

In 1957, rookie center Bill Russell joined the Boston Celtics, which already featured guard Bob Cousy and coach Red Auerbach, and went on to lead the franchise to eleven NBA titles in thirteen seasons. Center Wilt Chamberlain entered the league with the Warriors in 1959 and became a dominant individual star of the 1960s, setting new single-game records in scoring 100 and rebounding 55. Russell's rivalry with Chamberlain became one of the greatest rivalries in the history of American team sports. The 1960s were dominated by the Celtics. Led by Russell, Cousy, and Auerbach, Boston won eight straight championships in the NBA from 1959 to 1966. This championship streak is the longest in NBA history. They did not win the title in 1966–67, but regained it in the 1967–68 season and repeated in 1969. The domination totaled nine of the ten championship banners of the 1960s. Through this period, the NBA continued to evolve with the shift of the Minneapolis Lakers to Los Angeles, the Philadelphia Warriors to San Francisco, the Syracuse Nationals to Philadelphia to become the Philadelphia 76ers, and the St. Louis Hawks moving to Atlanta, as well as the addition of its first expansion franchises. The Chicago Packers (now Washington Wizards) became the ninth NBA team in 1961. From 1966 to 1968, the league expanded from 9 to 14 teams, introducing the Chicago Bulls, Seattle SuperSonics (now Oklahoma City Thunder), San Diego Rockets (who moved to Houston four years later), Milwaukee Bucks, and Phoenix Suns. In 1967, the league faced a new external threat with the formation of the American Basketball Association (ABA). The leagues engaged in a bidding war. The NBA landed the most important college star of the era, Kareem Abdul-Jabbar (then known as Lew Alcindor). However, the NBA's leading scorer, Rick Barry, jumped to the ABA, as did four veteran referees—Norm Drucker, Earl Strom, John Vanak, and Joe Gushue. In 1969, Alan Siegel, who oversaw the design of Jerry Dior's Major League Baseball logo a year prior, created the modern NBA logo inspired by the MLB's. It incorporates the silhouette of Jerry West, based on a photo by Wen Roberts. The NBA would not confirm that a particular player was used because, according to Siegel, "They want to institutionalize it rather than individualize it. It's become such a ubiquitous, classic symbol and focal point of their identity and their licensing program that they don't necessarily want to identify it with one player." The iconic logo debuted in 1971 (with a small change to the typeface on the NBA wordmark in 2017) and would remain a fixture of the NBA brand. The ABA succeeded in signing a number of major stars in the 1970s, including Julius Erving of the Virginia Squires, in part because it allowed teams to sign college undergraduates. The NBA expanded rapidly during this period. From 1966 to 1974, the NBA grew from nine franchises to 18. In 1970, the Portland Trail Blazers, Cleveland Cavaliers, and Buffalo Braves (now the Los Angeles Clippers) all made their debuts expanding the league to 17. The New Orleans Jazz (now in Utah) came aboard in 1974 bringing the total to 18. Following the 1976 season, the leagues reached a settlement that provided for the addition of four ABA franchises to the NBA, raising the number of franchises in the league at that time to 22. The franchises added were the San Antonio Spurs, Denver Nuggets, Indiana Pacers, and New York Nets (now the Brooklyn Nets). Some of the biggest stars of this era were Abdul-Jabbar, Barry, Dave Cowens, Erving, Elvin Hayes, Walt Frazier, Moses Malone, Artis Gilmore, George Gervin, Dan Issel, and Pete Maravich. The end of the decade, however, saw declining TV ratings, low attendance and drug-related player issues – both perceived and real – that threatened to derail the league.

The league added the ABA's three-point field goal beginning in 1979. That same year, rookies Larry Bird and Magic Johnson joined the Boston Celtics and Los Angeles Lakers respectively, initiating a period of significant growth in fan interest in the NBA. The two had faced each other in the 1979 NCAA Division I Basketball Championship Game, and they later played against each other in three NBA Finals (1984, 1985, and 1987). In the 10 seasons of the 1980s, Johnson led the Lakers to five titles while Bird led the Celtics to three titles. Also in the early 1980s, the NBA added one more expansion franchise, the Dallas Mavericks, bringing the total to 23 teams. Later on, Larry Bird won the first three three-point shooting contests. On February 1, 1984 David Sternbecame commissioner of the NBA. Stern has been recognized as playing a major role in the growth of the league during his career. Michael Jordan entered the league in 1984 with the Chicago Bulls, spurring more interest in the league. In 1988 and 1989, four cities got their wishes as the Charlotte Hornets, Miami Heat, Orlando Magic, and Minnesota Timberwolves made their NBA debuts, bringing the total to 27 teams. The Detroit Pistons won the back-to-back NBA Championships in 1989 and 1990, led by coach Chuck Daly and guard Isiah Thomas.Jordan and Scottie Pippen led the Bulls to two three-peats in eight years during the 1991–1998 seasons. Hakeem Olajuwon won back-to-back titles with the Houston Rockets in 1994 and 1995. The 1992 Olympic basketball Dream Team, the first to use current NBA stars, featured Michael Jordan as the anchor, along with Bird, Johnson, David Robinson, Patrick Ewing, Scottie Pippen, Clyde Drexler, Karl Malone, John Stockton, Chris Mullin, Charles Barkley, and star NCAA amateur Christian Laettner.The team was elected to the Naismith Memorial Basketball Hall of Fame, while 11 of the 12 players (along with three out of four coaches) have been inducted as individuals in their own right, and eleven players. In 1995, the NBA expanded to Canada with the addition of the Vancouver Grizzlies and the Toronto Raptors. In 1996, the NBA created a women's league, the Women's National Basketball Association (WNBA).

After the departure of James in free agency in 2018, the Cavaliers' streak of playoff and Finals appearances ended. The Warriors returned for a fifth consecutive Finals appearance in 2019 but lost to the Toronto Raptors, who won their first championship after acquiring Kawhi Leonard in a trade. The 2019–20 season was suspended indefinitely on March 11, 2020, due to the COVID-19 pandemic, after Utah Jazz center Rudy Gobert tested positive for the coronavirus. On June 4, 2020, the NBA Board of Governors voted to resume the season in a 22-team format with 8 seeding games per team and a regular playoffs format, with all games played in a "bubble" in Walt Disney World without any fans present. This era also saw the continuous near year-over-year decline in NBA viewership. Between 2012 and 2019, the league lost 40 to 45 percent of its viewership. While some of it can be attributed to "cable-cutting", other professional leagues, like the NFL and MLB have retained stable viewership demographics. The opening game of the 2020 Finals between the Los Angeles Lakers and Miami Heat brought in only 7.41 million viewers to ABC, according to The Hollywood Reporter. That is reportedly the lowest viewership seen for the Finals since at least 1994 when total viewers began to be regularly recorded and is a 45 percent decline from game one between the Golden State Warriors and Toronto Raptors, which had 13.51 million viewers a year earlier. Some attribute this decline to the political stances the league and its players are taking, while others consider load management, the uneven talent distribution between the conferences and the cord-cutting of younger viewers as the main reason for the decline.