The governing body announced the move Friday via its Twitter account.
Nestor, who's had a long and successful doubles career, had been playing with Sweden's Robert Lindstedt for much of this season. This week Nestor is teaming with India's Leander Paes at the Winston-Salem Open.
The 40-year-old from Toronto owns eight Grand Slam doubles titles with various partners, including the 2004 U.S. Open, and he won Olympic gold for Canada in 2000 with Sebastien Lareau.
Nestor and Pospisil, a 23-year-old from Vernon, B.C., have previously played doubles, most notably for Canada's Davis Cup team. Their victory over an Italian pair in April helped Canada advance to the semifinals, where they'll meet Serbia next month.
Nestor and Pospisil also played together at last year's London Olympics, where they lost in the second round.
Pospisil is also entered in the U.S. Open singles draw.