It's a two-year contract, according to a source.
Ihedigbo has been negotiating with the Lions since late last week and agreed to terms tonight.
"It's Agreed upon! I'm a @DetroitLionsNFL," Ihedigbo wrote on his Twitter account. "I'm excited for what's ahead and helping this team achieve greatness!!! #HeartOfALion #LionsNation."
Ihedigbo, 30, had a career-high 101 tackles and three interceptions last season.
He visited the Lions last week.
Now, he's reunited with Lions defensive coordinator Teryl Austin, his secondary coach the past two seasons in Baltimore, and Lions head coach Jim Caldwell, the former Ravens offensive coordinator.
Ihedigbo recorded two forced fumbles and one fumble recovery and finished second on the Ravens in tackles behind middle linebacker Daryl Smith.
For his career, Ihedigbo has registered 244 career tackles, six sacks and three forced fumbles.