In a corresponding move, left-hander Tim Mayza was placed on the 60-day injured list, which opened up a spot on the Blue Jays’ 40-man roster.
Mayza is undergoing Tommy John surgery after injuring his left ulnar-collateral ligament over the weekend.
Toronto will be Dull’s fourth MLB team this season after he was previously designated for assignment by the Oakland Athletics, San Francisco Giants and Yankees.
The 29-year-old has a combined 13.50 earned-run average over 11.1 innings this season with 12 strikeouts and seven walks.
Dull last appeared was on Sept. 14 against the Blue Jays when he allowed two earned runs on three hits in one inning of work.