The 21-year-old Spaniard is set to compete for a second US Open title in three seasons when the year's final Grand Slam event begins on August 26.
"It has been a very long season for me and due to the accumulation of matches and fatigue, I will not be able to play in Montreal this year," Alcaraz said. "I was really looking forward to coming back to Canada and enjoying the love of the fans. Hope to see everyone in Montreal in the years to come."
Third-ranked Alcaraz, 37-6 this season, still has a Masters tuneup event later this month in Cincinnati, where he lost in the final last year.
Alcaraz lost in his opener in Canada in 2022 and reached the quarter-finals last year.
"We're obviously disappointed that Carlos will not be with us this year," tournament director Valerie Tetreault said.
"We were also well aware that he has had a busy summer and is currently on a quest to achieve a major feat: winning consecutive French Open, Wimbledon and Olympic titles."
Organizers also announced the withdrawals of 26th-ranked Francisco Cerundolo and 30th-ranked Jiri Lehecka.
The moves allowed American Marcos Giron, ranked 38th, Australia's 63rd-ranked Alexei Popyrin and Italy's 48th-ranked Flavio Cobolli into the Montreal main draw.
