The 35-year-old defender has moved on a free transfer after his one-year contract at Brighton and Hove Albion expired and becomes the second arrival in the space of as many days following the signing of goalkeeper Ben Hamer.
With over 400 appearances under his belt at club level, Upson adds plenty of experience to the Leicester ranks and was capped 21 times for England.
The majority of Upson's career has been spent in the top flight with the likes of Birmingham City, West Ham and Arsenal and he is relishing the chance of being back in the Premier League after Leicester romped to the Championship title.
"This is too good an opportunity to turn down," Upson told the club's official YouTube channel.
Nigel Pearson's men had the second-best defensive record in the Championship this season behind only Burnley, who also sealed automatic promotion along with Leicester.
