The Tigers announced the move Sunday.
After going 14-9 as a 20-year-old rookie last year, Porcello is 4-7 with a 6.14 ERA this season.
The Tigers did not immediately announce who would replace Porcello on the roster or in the rotation. Because of an off-day, Detroit wouldn't need a fifth starter until June 29.
One possibility is Enrique Gonzalez, who was recently called up to pitch long relief and had gone 4-5 in 11 starts in Triple-A before reaching the majors.