Marion, whose lobbying to play in Wednesday's home loss to the Los Angeles Lakers was rebuffed by coach Rick Carlisle and head athletic trainer Casey Smith, fully participated in Friday morning's shootaround in San Antonio.
"I expect him to play tonight," Carlisle told reporters before the game.
Marion told reporters after the shootaround that he feels "a whole lot better."
Marion's return gives Dallas back its top perimeter player. He might be matched up against Spurs guard Manu Ginobili, who started because point guard and leading scorer Tony Parker has been ruled out with a hamstring strain sustained Wednesday night against Minnesota.
Dallas missed starting center Brendan Haywood for a fourth consecutive game with a sprained right knee. Guard Delonte West was also out and is likely to miss another week or more with a fractured right ring finger.
Marion last played on March 13 against Washington. He had played through pain in his left knee for weeks and complained before the All-Star break of being physically worn down.
Since West was sidelined on Feb. 15, Marion has been the Mavs' designated defender against the opposing team's best player, whether a wing player or a point guard.
Before sitting out last Thursday's game against Charlotte, Marion said his knee wasn't getting worse, but that he was concerned because it was causing discomfort and wasn't getting better.
A thorough medical exam Monday revealed no structural damage in his knee.
Marion is Dallas' third-leading scorer at 11.2 points per game and is tied for the team rebounding lead with Dirk Nowitzki at 6.8 per game. In the 11 games he's played since the All-Star break, he is averaging 8.9 points and 7.8 rebounds per game.