Grimes, who had been the franchise player for the Atlanta Falcons last season, has been hobbled by injuries but was expected to step into the starting job that opened up when Sean Smith signed with Kansas City.
Grimes spent six seasons in Atlanta and made the Pro Bowl in 2010.
"His experience and history of play making production will add a great deal to our secondary," Dolphins General Manager Jeff Ireland said in a statement.
Miami has been active in the off-season free agent market. They added coveted wide receiver Mike Wallace this winter and signed defensive lineman Vaughn Martin this week.
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