Wilhoite's deal is worth up to $1.55 million with $500,000 guaranteed, per a league source.
Wilhoite could get a chance to compete for the starting strongside linebacker spot, which was occupied by Mike Morgan last season. Morgan is an unrestricted free agent and has not been signed.
Head coach Pete Carroll said after the season that the Seahawks need to add more linebacker depth behind starters Bobby Wagner and K.J. Wright. In addition to Wilhoite, the team signed Arthur Brown last week. And according to a league source, the Seahawks also have agreed to terms with veteran Terence Garvin.
Wilhoite, who turned 30 in December, ended up playing much more than originally projected last season for the San Francisco 49ers because of injuries at linebacker.
He's a veteran with experience and special-teams ability, but the Niners are poised to go in a different direction in new coordinator Robert Saleh's 4-3 defense.
Wilhoite had 256 tackles, three interceptions, two forced fumbles and a half-sack in 65 games (36 starts) for the 49ers.
ESPN's Nick Wagoner contributed to the report.