The Celtics finalized a four-year, $5.5 million contract with rookie point guard Demetrius Jackson on Tuesday, according to a league source. The fourth year of the deal will be a team option, the source said.

Jackson, the 45th overall pick of last month’s draft, averaged 5.2 points, 2.0 rebounds, and 1.2 assists over eight summer league games with the Celtics this month. He joins a deep backcourt of point guards that includes Isaiah Thomas, Terry Rozier, and Marcus Smart.

Jackson is a strong, quick, and athletic guard who many draft analysts slotted as a late first-round pick. His 3-point shooting did fall from 42.9 percent as a sophomore at Notre Dame to 33.1 as a junior last year, when he was tasked with handling the ball more frequently.


Last season, Celtics second-round pick Jordan Mickey signed a four-year, $5 million deal.

Adam Himmelsbach can be reached at adam.himmelsbach@globe.com. Follow him on Twitter at@adamhimmelsbach.