In a game on June 27, 2017, the Washington Nationals went seven-for-seven in stolen bases against Montero. After the game, he said that it was the fault of pitcher Jake Arrieta, who took too long to deliver his pitches and failed to hold the runners at their bases. The next day, the Cubs designated Montero for assignment. Montero apologized to Arrieta after the incident. It was reported this was done because management felt he was a disruption in the clubhouse; Anthony Rizzo considered Montero's comments to the media unprofessional.
On July 3, 2017, Montero was traded to the Toronto Blue Jays for cash considerations or a player to be named later. He was placed on the disabled list with a groin strain on August 4. After a one-game rehab assignment with the Triple-A Buffalo Bisons, Montero was activated on August 16.
Montero is married to Vanessa. Together, in 2017, the couple created the Miguel and Vanessa Montero Foundation, which aims to provide medical assistance to seriously ill children from Venezuela. On May 1, 2017, Montero and his wife both became United States citizens.