The seven-year veteran told NFL Network insider Jason La Canfora Friday that he was already in Seattle and would be playing on Sunday when the Seahawks travel to San Francisco.
The Seahawks released defensive tackle Landon Cohen Saturday to make room for Hargrove on the active roster.
Hargrove signed with the Philadelphia Eagles in August before he was released during training camp. He was originally drafted by the St. Louis Rams in the third round of the 2004 NFL Draft. Following three years in St. Louis, he spent two seasons with the Buffalo Bills and the last two with the New Orleans Saints, where he started 30 games.
Hargrove has started 25 of 87 career games and posted 16.5 career sacks.