The 24-year-old will fill the void left on the club's 25-man roster following the departure of Brad Mills, who was optioned to Triple-A on Thursday.
Carreno made 23 starts this season for Double-A New Hampshire, but will be used exclusively out of the bullpen in Toronto.
"He has an above-average slider," Blue Jays manager John Farrell said. "His velocity has been a tick above average -- he's in that 91-93-mph [range].
"The one thing that he showed in Spring Training, much like Henderson [Alvarez] and Luis [Perez] -- they come to this level and [have] not been intimidated. He had very good mound presence in Spring Training, and we're looking forward to seeing the same."
Carreno posted a 7-9 record with a 3.41 ERA in 134 2/3 innings this year. He struck out 152 with 68 walks in his first season above Class A.
Farrell said the club will ease the Dominican native into his new role because of his lack of experience in relief. Carreno has made just one appearance this season out of the bullpen, but pitched exclusively in that role during Spring Training.
"We know there's going to be a little bit of a transition," Farrell said. "We'll finish off an inning with a guy that has been a reliever ... just to buy enough time to make sure a starter coming into the bullpen has ample time to get loose."