Drew Rosenhaus, Shelton's agent, provided the update to NFL Network's Ian Rapoport.
The 26-year-old University of Washington product is coming off a strong season with the Pats. He recorded 61 total tackles, three sacks and a forced fumble across 16 appearances (14 starts) in 2019.
It represented a much-needed bounce-back campaign for the 6'2", 345-pound lineman, who only tallied 21 tackles while mostly working in a reserve role for New England in 2018.
"Building on the experiences he had last year has really given us a good level of play inside," head coach Bill Belichick told reporters in October. "His role actually has expanded from what it was last year because he's been able to do more things, and he's doing the things we've asked him to do at a good level. He's been productive and versatile. He's been big. Done a good job for us."
Shelton spent his first three pro seasons with the Cleveland Browns, who selected him with the 12th overall pick in the 2015 draft. He tallied 128 tackles and 1.5 sacks in 46 games (44 starts) in Cleveland.
He'll likely slot in alongside fellow free-agent signing Nick Williams, who spent the past two seasons with the Chicago Bears, as the Lions' starters at defensive tackle. Williams is prepared to sign a two-year, $10 million contract with Detroit, per Adam Caplan of Sirius XM NFL Radio.
They should help bolster a Lions defense that ranked 21st in rushing yards allowed per game (115.9) last season.