The 32-year-old Williams has played in 128 regular season games with 99 starts, having spent time with Minnesota (2002-05), Jacksonville (2006-08) and Atlanta (2009-10). He has 19 career interceptions and played in all 16 games last season.
"He is a veteran who knows how to play. He has been a good physical blitzer and we are in need of someone who can learn quick and go from there," Giants coach Tom Coughlin said.
Cornerback Terrell Thomas tore his anterior cruciate ligament during Monday's preseason win over the Chicago Bears. That incident came two weeks after rookie cornerback Prince Amukamara suffered a fracture of the fifth metatarsal bone in his left foot.