Pees takes over for the departed Chuck Pagano, who was named head coach of the Indianapolis Colts earlier this week.
"It's an incredible opportunity to be a defensive coordinator in this league, but it's especially humbling to be one for the Ravens," Pees said. "We've got a great group. It is a unique group, one of the most unique groups I've ever been around. They've got their own style, their own personality. It's not going to change and I'm not going to change it."
The 62-year-old will guide a defense which finished the 2011 season third in the NFL in points allowed (266) and yards allowed per game, also racking up the most sacks (48) in the AFC.
"The tradition of this defense will continue, and it will flourish and it will get even better," Baltimore head coach John Harbaugh said.
Pees also coached the linebackers at New England from 2004-05 before taking over the coordinator's position -- which he held until joining the Ravens following the 2009 season. The Patriots' defense allowed less than 20 points per game in every season under Pees' guidance.