The 48-year-old Undlin has been in the NFL since 2004, when he was a defensive assistant with the New England Patriots -- and worked with Lions head coach Matt Patricia.
Patricia had started with the Patriots as an offensive assistant in 2004.
Since then, Undlin has been in Cleveland, Denver, Jacksonville and then Philadelphia, where he has been defensive backs coach since 2015.
With the Eagles, Undlin worked under former Lions head coach Jim Schwartz.
Undlin played safety at Cal Lutheran from 2000 to 2004.
With Undlin's hiring, Detroit has filled three of its eight main openings -- hiring Brayden Coombs as special teams coordinator and promoting Hank Fraley to offensive line coach along with bringing Undlin in. The Lions still are looking for on-field assistants for tight ends, linebackers and defensive backs along with their two top strength-and-conditioning staff members.