The 35-year-old, who names Arsenal and West Ham among his previous teams and whose contract at Brighton recently expired, says the offer was "too good" to turn down
The 35-year-old defender has moved on a free transfer after his contract at Brighton 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 - including the likes of Arsenal and West Ham - Upson adds plenty of experience to the Leicester ranks and was capped 21 times for England.
"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.