The Boston Celtics are off the market ... the 18-time NBA champions have been scooped up for a whopping $6.1 billion -- making it the largest sale for a sports franchise in North American history.

Bill Chisholm -- the managing partner at Symphony Technology Group -- was the man to submit the winning bid for the 2024 NBA champs. The $6.1 billion price tag surpasses the number previously set by Josh Harris when he purchased the Washington Commanders in 2023 for $6.05 billion.

The Celtics hit the market not long after they defeated the Dallas Mavericks in the 2024 NBA Finals. Their current owner -- 63-year-old Wyc Grousbeck -- announced plans to sell his stake for "estate and family planning considerations." Grousbeck intended to sell 51% of the team by late 2024 or early 2025, with the rest closing in 2028.

He is expected to remain on as team governor until 2028. The Grousbeck purchased the Cs back in 2002 for $360 million.

Chisholm -- a Dartmouth graduate -- grew up 40 minutes outside of Beantown ... and is said to be a longtime fan of the squad.

Neither the Celtics or Chishom have commented on the sale.