The agreement for the one-time Northwestern all-America linebacker who has coached the Wildcats to three straight winning seasons replaces a seven-year contract signed in 2009. Financial details were not disclosed.
Fitzgerald's record with Northwestern is 34-29 over five seasons.
"It is paramount for Northwestern football to have great leadership," Athletic Director Jim Phillips said. "There is no better individual to lead Chicago's Big Ten Team than Pat. He is a tremendous teacher and truly values developing young men athletically, academically and socially."
Fitzgerald played for the Wildcats in the 1990s, helping them win the Big Ten championship in 1995 and tie for the title in 1996. He was inducted into the College Football Hall of Fame in 2008.
"I'm humbled and appreciative of the commitment shown by Northwestern," Fitzgerald said after the new contract was signed. "With this support from our administration, our coaches and players expect to compete for championships."