The second-year pro was placed on injured reserve Tuesday, two days after he was hurt making a tackle on the opening kickoff of Sunday's 16-6 loss at Carolina.
His first concussion happened during an Oct. 18 game against the Panthers, when he was hit trying to make a fair catch on a punt.
Smith was a Pro Bowl kick returner as a rookie. He's second in the NFL in kickoff return average (29.1 yards) and 10th in punt return average (10.1) this season.
The Bucs promoted running back Kareem Huggins from the practice squad to fill the roster opening.
The move was announced on the team's day off.
On Monday, coach Raheem Morris was asked if the Bucs would take a cautious approach with Smith's second concussion.
"That's a no-brainer. ... I don't want to speak out of turn but I'm sure there will be with what's going on with concussions and our society as far as the NFL," Morris said.