The 30-year-old is a three-time Pro Bowler, but he is coming off three largely injury-plagued, disappointing seasons with the New Orleans Saints.
Byrd missed 15 games combined during his first two seasons with the Saints before rebounding to appear in all 16 during the 2016 campaign.
He registered 82 tackles with three passes defended and two interceptions, which marked his best production since 2013 with the Buffalo Bills.
Byrd was once among the NFL's premier ball hawks, as he intercepted 22 passes during his first five NFL seasons. He reeled in just three picks in the past three years, however.
Provided the Niners sign Byrd, he could start alongside Eric Reid, allowing Jimmie Ward to focus more on cornerback, which is a major position of weakness for the team.
Even if Byrd serves in a reserve role, he would add much-needed depth to a secondary that surrendered the sixth-most passing touchdowns in the NFL last season.