The veteran Crusader and All Black has taken up a two-year deal with Section Pau in the South of France, where he will link up with younger brother Luke who joined the Top 14 club in 2019.
"Hannah, the kids and myself are going to head to France to play with my little brother Luke," Whitelock said.
"I've had the privilege of playing with family for Canterbury, the Crusaders and the All Blacks at different stages and this opportunity was too good to pass up."
Whitelock debuted for Canterbury in 2008 and the Crusaders in 2010.
He has played 176 games for the franchise, second only to Wyatt Crockett's record 203 caps.
The All Black has been a part of six championship campaigns and captained the side for several seasons.
"I want to say thank you to everyone for their support through the years, it's been absolutely immense," Whitelock said.
Crusaders coach Scott Robertson said Whitelock was an iconic rugby player who had taught him a lot throughout his coaching career.
"He's iconic because of his consistency in form at all levels, spanning three decades and being at the top of his game throughout that time," Robertson said.
He said Whitelock's game understanding and "his ability to win big moments in big games" set him apart.
"He has an innate ability to connect the group and know what needs to be said at the right time on and off the field. He's a great mate and a pleasure to coach."
Whitelock will join Pau after the Rugby World Cup.