Earlier Thursday, ESPN reported the Lions renegotiated Spence's contract, cutting his 2018 salary from $3.15 million to $2.575 million.
Miami will assume Spence's contract, a three-year contract that Spence inked with the Lions as a free agent in 2017.
In exchange for Spence, the Lions will receive an undisclosed 2019 draft pick. He was expected to be the primary backup at defensive tackle, but wound up starting 11 games after the loss of Haloti Ngata in the heart of the defense.
An elbow injury to defensive tackle Haloti Ngata ended his season in Week 5 a year ago and Spence moved into a starting role with career highs of 39 tackles, three sacks and a forced fumble.
Spence, 6-foot-1 and 307 pounds, has been on the trade block in Detroit because the team hired former New England Patriots defensive coordinator Matt Patricia.
Fans hoping the Detroit Lions got a bonanza in return for a situational defensive tackle they gave a pay cut before trading...will be disappointed.
The Lions moved up in the fourth round to take Alabama defensive end Da'Shawn Hand, who has the ability to play inside. Only A'Shawn Robinson returns from that mix, along with reserves Jeremiah Ledbetter and Christian Ringo.