Terms of the deal were not disclosed, but ESPN.com's Marc Stein reported that the Spurs were able to sign Van Exel without cutting into the estimated $2.5 million they have offered free-agent swingman Michael Finley for next season.
The 33-year-old Van Exel is coming off a season in Portland in which he posted some of the lowest numbers of his 12-year NBA career. He averaged 11.1 points, 3.0 rebounds and 4.3 assists in 53 games for the Trail Blazers.
For the Spurs, however, Van Exel's scoring ability might help a team offense that sometimes has trouble putting points on the board.
"We are very happy to have Nick as a part of our team," coach Gregg Popovich said. "His leadership and skills will be assets at both ends of the floor."
The 6-foot-1 guard has averaged 15.1 points, 3.0 rebounds and 6.9 assists in 815 regular-season games. He has shot 40.5 percent from the field and made nearly 36 percent of his 3-pointers.
His 1,473 3-pointers rank him behind Ray Allen (1,486) among active players.
Van Exel was selected out of Cincinnati in the second round of the 1993 draft by the Los Angeles Lakers. He played there for five years before going to Denver for four seasons. He also played for Dallas and Golden State before joining the Trail Blazers last season.
Finley is expected to decide early this week whether to sign with the Spurs, Suns, Heat or Timberwolves.
Information from The Associated Press was used in this report.