Ridgewell, 34, was a free agent having recently left Portland Timbers after more than four years with the MLS side. Before heading to the US, the centre back had racked up almost 350 appearances during lengthy spells with Aston Villa, Birmingham City and West Bromwich Albion.
He has also previously had loan spells with Bournemouth, Wigan Athletic and Brighton & Hove Albion and is now looking forward to being back in English football with the Tigers.
“I’m delighted to sign having spent the last few days up here training with the boys,” said Ridgewell.
“It’s nice to be back in English football and back in the Championship. The boys have really welcomed me and this is a team looking to move in the right direction. They’ve been on a good run up until last weekend, and it’s a group that is prepared to work hard for each other and hopefully I can use my experience to help the side to push on a little bit more.”