The Boston Celtics (/ˈsɛltɪks/ SEL-tiks) are an American professional basketball team based in Boston, Massachusetts. The Celtics compete in the National Basketball Association (NBA) as a member of the league's Eastern Conference Atlantic Division. Founded in 1946 as one of the league's original eight teams, the team play their home games at TD Garden, which they share with the National Hockey League (NHL)'s Boston Bruins. The Celtics are regarded as the most successful basketball team in NBA history; the franchise has won the most championships in the NBA with 17 (accounting for 23.9 percent of all NBA championships since the league's founding), and currently hold the record for the most recorded wins of any NBA team.

The Celtics have a notable rivalry with the Los Angeles Lakers, who have won 16 NBA championships, second behind the Celtics. The rivalry was heavily highlighted throughout the 1960s and 1980s. The franchise has played the Lakers a record 12 times in the NBA Finals (including their most recent appearances in 2008 and 2010), of which the Celtics have won nine. Four Celtics players (Bob Cousy, Bill Russell, Dave Cowens and Larry Bird) have won the NBA Most Valuable Player Award for an NBA record total of 10 MVP awards. Both the nickname "Celtics" and their mascot "Lucky the Leprechaun" are a nod to Boston's historically large Irish population.

The Celtics' rise to dominance began in the late 1950s, after acquiring center Bill Russell in a draft day trade in 1956, who would become the cornerstone of the Celtics dynasty. Led by Russell and superstar point guard Bob Cousy, the Celtics won their first NBA championship in 1957. Russell, along with a talented supporting cast of future Hall of Famers including John Havlicek, Tom Heinsohn, K. C. Jones, Sam Jones, Satch Sanders, and Bill Sharman, would usher the Celtics into the greatest period in franchise history, winning eight consecutive NBA championships throughout the 1960s. After the retirement of Russell in 1969, the Celtics entered a period of rebuilding. Led by center Dave Cowens and point guard JoJo White, the Celtics returned to championship caliber, winning two NBA titles in 1974 and 1976. The Celtics again returned to dominance in the 1980s. Led by the "Big Three" that featured Larry Bird, Kevin McHale, and Robert Parish, the Celtics won the 1981, 1984, and 1986 championships. After winning 16 championships throughout the 20th century, the Celtics, after struggling through the 1990s, rose again to win a championship in 2008 with the help of Kevin Garnett, Paul Pierce, and Ray Allen in what was known as the new "Big Three" era.

The Celtics returned to the championship in 2010, only to lose to the Lakers in a seven game series. In the following season the Celtics were unable to attain the same degree of prowess.

By the start of the 2013 season none of the "Big Three" were still signed - Ray Allen having already left a season prior - ushering in a new era for the team. With the help of newly hired head coach Brad Stevens, the team began rebuilding. In his second season, Stevens led the Celtics on a return to the playoffs in 2015. During the following season, the Celtics clinched the top seed in the Eastern Conference, but were eliminated in the Conference Finals. This prompted an aggressive rebuild in 2017, where the team acquired All-Stars Kyrie Irving and Gordon Hayward. However, the pair struggled with injuries throughout the 2017–18 season, and the team was again defeated in the Eastern Conference Finals.