To make room on the 40-man roster, the Twins placed starting pitcher Scott Baker on the 60-day disabled list. Baker needs surgery on his right elbow and is out for the season.
Thomas had no plate appearances with the Tigers this season, but appeared in three games before being designated for assignment on April 12. He has a career batting average of .253 in 145 games with the Tigers.
Thomas is expected to join the Twins in time for Sunday's game against Texas. Minnesota will make a corresponding roster move before adding him to the 25-man roster.