Collmenter was one of five roster moves the D-backs made as they selected left-handers Adam Loewen and Steve Hathaway from Triple-A Reno and optioned lefty Zac Curtis to Double-A Mobile and infielder/outfielder Michael Freeman was designated for assignment.
By designating Collmenter and Freeman, the D-backs freed up two spots on the 40-man roster, which were filled by Loewen and Hathaway.
A player designated for assignment can either be traded, released, or outrighted to the Minors. Collmenter could refuse an outright assignment and then would be a free agent.
"It's a tough day for the organization," D-backs manager Chip Hale said of the move involving Collmenter. "This is a guy who has been our No. 1 starter. He has started a game in the playoffs and won it for this organization. It's just a tough day. It was time for us to move on. He understands the business of the game."
Hale said the team selected Loewen and Hathaway because the team wanted to have a pair of lefties in the bullpen.
Loewen became the first Canadian-born pitcher to appear in a game for the D-backs when he tossed a scoreless inning Saturday night.
Collmenter, 30, was the team's Opening Day starter last year. He was selected by the D-backs in the 15th round of the 2007 Draft and burst on the scene in 2011 when he won 10 games and then helped the D-backs stave off elimination by winning Game 3 of the National League Division Series.
"It's been great," Collmenter said of his time in Arizona. "Just to have the opportunity in the first place in an organization that did so much on the field as well as off. Obviously getting a lot of guys back come September so it will be fun to see this team play with everybody they thought they would have had coming out of Spring Training. So good things are still happening with this club."
As for Collmenter, his immediate plans included a trip to the beach with some friends who were in town visiting him.
Steve Gilbert has covered the D-backs for MLB.com since 2001. Follow him on Twitter @SteveGilbertMLB. This story was not subject to the approval of Major League Baseball or its clubs.