The Cleveland Cavaliers and the Boston Celtics, two teams already primed to contend for supremacy in the N.B.A.’s Eastern Conference next season, agreed on Tuesday to one of the biggest deals of a frantic off-season when they swapped All-Star point guards.

The Cavaliers sent Kyrie Irving, one of the most dynamic playmakers in the league, to the Celtics in exchange for a hefty package headlined by Isaiah Thomas, a two-time All-Star. In exchange for Irving, Boston also agreed to send forward Jae Crowder, center Ante Zizic and a 2018 first-round draft pick to Cleveland.

And as if the deal itself were not intriguing enough, the Celtics and the Cavaliers are scheduled to face each other in Cleveland to open the N.B.A. season on Oct. 17.

It was a seismic trade in an already busy summer for the N.B.A., in which a raft of stars relocated to new teams. Among them was Gordon Hayward, an All-Star forward who left the Utah Jazz in free agency last month to sign a lucrative contract with the Celtics, who have reshaped their roster after finishing with the top record in the conference last season.