Hummel's agency, Priority Sports, announced the agreement on Monday night. He averaged 3.4 points in just over 12 minutes per game as a rookie last season.
Bringing Hummel back gives president and coach Flip Saunders a shooter off the bench with the ability to play multiple positions.
It's also the next chapter in a nice comeback story for Hummel, who has overcome major knee injuries in college at Purdue to make the NBA.
When completed, the deal will give the Timberwolves 14 players under contract for next season. That leaves them with one open roster spot as they continue to address areas of need.