Maine Red Claws chairman and principal owner Bill Ryan Jr., left, and team president Dajuan Eubanks are shown in 2015 after the franchise installed new seating at the Portland Expo. Despite the $360,000 investment, attendance for the team has dropped every season since. Whitney Hayward/Staff Photographer

The Boston Celtics announced Thursday that they have reached an agreement to purchase the Maine Red Claws, reflecting a trend that has seen NBA teams buy their minor league affiliates.

The teams have been affiliated since the Red Claws’ inaugural season in 2009-10. In 2012, the Celtics became the team’s sole affiliate, assuming control of all basketball operations while the Red Claws maintained the business side of the G League franchise.

“Over the years, the G League and the Red Claws have become increasingly important to the Celtics and our player development efforts,” Celtics president Rich Gotham said in a news release. “We look forward to deepening both our ties with basketball operations and within the community, and further bringing the Celtics experience to fans in Maine.”

The Red Claws, who play at the Portland Expo, were one of the few remaining independently owned G League franchises. Bill Ryan Jr. and his father, former TD Bank chairman and CEO Bill Ryan, have been among the Red Claws’ owners since the start.

“The Celtics have been great partners with the Red Claws over the last 10 seasons,” Bill Ryan Jr., the Red Claws chairman and principal owner, said in the release. “We are thrilled that the Celtics will continue to build upon the success we’ve had in making the Red Claws an important part of the Maine community.”

The Red Claws have a five-year lease with the city of Portland to play at the Expo through the 2023-24 season, with a five-year option to extend the lease through 2029, according to city spokeswoman Jessica Grondin. The City Council approved the extension in 2015.

But it’s unclear what the long-term relationship between the Celtics and Portland might be. The Celtics and Red Claws said in the news release they will not have further comment until the completion of the purchase, which must be approved by the G League.

In some cases, NBA teams have purchased their minor league affiliate and moved it closer geographically. For example, the NBA’s Oklahoma City Thunder purchased the Tulsa 66ers in 2008 and moved the franchise 107 miles to Oklahoma City six years later, rebranding them as the Blue.

After selling out every home game their first two seasons, the Red Claws have seen their attendance drop every season since. In 2015, the Red Claws spent $360,000 on renovations to the Expo, including new stadium-style chairs and back rests for bleacher seats. Still, attendance has dropped from more than 73,000 annually in the first two years to a franchise-worst 44,633 last winter.

Portland City Manager Jon Jennings said the sale is a good step in the relationship between the Celtics and the city. Jennings was the Red Claws’ general manager and president for the team’s first three seasons and also a former assistant coach for the Celtics.

“As the person who worked with Bill Ryan Jr. to originally create the Maine Red Claws in 2009 and who worked for the Boston Celtics for 12 years, I am thrilled to know the Boston Celtics have purchased the team,” he said. “It is exciting to know the Celtics have decided to fully invest in the Red Claws and be a long-term partner with the city.”

The Celtics’ purchase reflects a growing business model over the past decade. At least 24 NBA teams now own their G League affiliates. Several others employ a hybrid franchise where the NBA club controls the basketball aspects of the franchise and the local owners maintain the business side.

Under the terms of their affiliation with the Red Claws, the Celtics have run the basketball side of operations. They select the coaching staff, which plays the primary role in player development. Two players on G League rosters are designated as two-way players, who can spend up to 45 days with the NBA team and the rest of the season in the G League. Last season, the two-way players in Portland were P.J. Dozier (six games with the Celtics) and R.J. Hunter (one game).

The Red Claws have run the business side. Under terms of the team’s lease with the city, the franchise has received net proceeds from parking and concessions and has been allowed to sell advertising and sponsorships. The Expo is currently being used to shelter migrants and the city is working feverishly to find housing for them by Aug. 15 to make the arena available as part of its commitment to the team.

“We are pleased the Boston Celtics have agreed to purchase the Maine Red Claws,” G League spokeswoman Joanna Shapiro said. “The Celtics and Red Claws have had a successful partnership for many seasons and we’re excited to watch the relationship continue to grow.”