Frazier, 56, was head coach of the Vikings from 2010-13 (interim for part of 2010) and their defensive coordinator from 2007-10. He enters his 18th NFL coaching season after spending the 2014-15 season as defensive coordinator for the Tampa Bay Buccaneers.
Frazier is the ninth former NFL head coach Harbaugh has added to his staff since joining the Ravens in 2008. The others are: Cam Cameron (Miami Dolphins), Al Saunders (San Diego Chargers), Jim Zorn (Washington Redskins), Jim Caldwell (Indianapolis Colts), Steve Spagnuolo (St. Louis Rams), Gary Kubiak (Houston Texans), Marty Mornhinweg (Detroit Lions) and Marc Trestman (Chicago Bears).
Frazier joins quarterbacks coach Mornhinweg and offensive coordinator Trestman now on Harbaugh's staff.
The Ravens also announced Friday night they hired Joe Cullen, who will be the team's defensive line coach. Clarence Brooks, the Ravens' longest-tenured coach, will continue to work as a senior defensive assistant, including helping with the defensive line. Brooks joined the Ravens in 2005.
Cullen, a 10-year NFL coaching veteran, joins the Ravens after serving as defensive line coach for the Buccaneers (2014-15), Cleveland Browns (2013), Jacksonville Jaguars (2010-12) and Lions (2006-08).
Chris Hewitt, a Ravens' assistant since 2012, will continue to coach defensive backs. Matt Weiss, a member of Harbaugh's staff since 2009, will now coach linebackers. Weiss focused on coaching the team's cornerbacks in 2015.
Scott Cohen, who was a consultant for the Ravens' coaching staff and Harbaugh last season, has been named coaching assistant/opponent analysis. Cohen previously was the senior personnel advisor for the Buccaneers for two seasons (2013-14). He also served as the New York Jets' assistant general manager (2008-12) and the Philadelphia Eagles' director of pro personnel (2001-07) after working as the Jaguars' assistant director of pro scouting (1999-2000).
"We were excited when coaches Frazier and Cullen became available in the last few days, and we are pleased to keep Scott, who was being pursued by other teams," Harbaugh said in a statement.