The Championship outfit have added the central defender to their ranks following his release from Norwich City, making him Nigel Pearson fifth acquisition of a busy summer
The central defender becomes the club's fifth arrival of the summer after Marko Futacs, Jamie Vardy, Matty James and Ritchie De Laet joined Nigel Pearson's side.
Whitbread will be in contention to fill the void left by centre-back Matthew Mills, who switched to Bolton Wanderers earlier in the summer.
"Zak is a player of great quality that will add a huge amount of experience to the squad; I'm delighted we've been able to bring him here," Pearson told the club's official website.
"He will be a great character to have as part of the group and, having only won promotion from the Championship a little over a year ago with Norwich, he knows exactly what it takes to get out of this division.
"We're really pleased with the business we have been able to do so far this summer, we have made some valuable additions to the squad and Zak certainly falls into that category."
Whitbread, 28, started his career at Liverpool and also played for Millwall before moving to Carrow Road where he made 44 appearances and scored once.