Buchholz, 28, is scheduled to report to Toronto. He made seven relief appearances for the Rockies this season, going 1-0 with a 4.50 ERA after missing all of 2009 recovering from Tommy John surgery.
He spent the early part of the season in the Minor Leagues and was 1-1 with a 5.06 ERA in 21 games, including one start, for Triple-A Colorado Springs.
Buchholz has appeared in 133 Major League games, including 27 starts over four seasons -- he was with the Houston Astros in 2006 and played for the Rockies since '07 -- and is 19-21 with a 4.42 ERA.
In another move, the Blue Jays recalled left-hander Brad Mills from Triple-A Las Vegas, where he was 8-6 with a 4.97 ERA in 20 starts. He was 1-0 with a 5.28 ERA in three starts with Toronto.