The Lions sent Diggs and a 2021 seventh-round draft pick to the Seahawks in exchange for a 2020 fifth-round selection, according to the team.
Diggs, who was a starter for the Lions since 2017 at slot cornerback and safety, helped anchor Detroit's secondary over the last two seasons, along with Pro Bowl cornerback Darius Slay.
Diggs, 26, was a Pro Bowl alternate last year after he recorded 78 total tackles, three interceptions, eight passes defensed and one defensive touchdown.
This season, Diggs has missed one game and portions of another because of a hamstring injury. In five games, he has notched 20 combined tackles for the Lions.
Seahawks starting strong safety Bradley McDougald missed Sunday's game against the Baltimore Ravens due to back spasms and backup Lano Hill could miss at least another game with an elbow injury, creating an opening in Seattle's defensive backfield.
Rookie second-round pick Marquise Blair made his first career start alongside free safety Tedric Thompson against the Ravens. Baltimore quarterback Lamar Jackson completed 9-of-20 passes for 143 yards, including a 50-yard completion over Thompson.
Detroit selected Diggs in the sixth round of the 2015 NFL Draft out of Texas. He originally signed a three-year, $20.4 million contract extension with the Lions last season.