Stewart spent his first nine years on campus coaching defensive backs before he was promoted to defensive coordinator in 2010.
The Bulldog defense ranks last in the WAC in scoring defense and second to last in total defense.
Current linebackers coach and former Bulldog player Tim Skipper will take over the defensive coordinator duties for the rest of the season.
Head coach Pat Hill says Stewart will help in recruiting and film breakdown of upcoming opponents until his contract ends.
Fresno State head football coach Pat Hill has issued the following statement Sunday night.
"Today, Randy Stewart has decided to step down as defensive coordinator at Fresno State. Together, we looked at our defense over the past year and a half and he said we were making progress, but it was not going as we had hoped over the past four weeks and a change was needed.
"Randy has been a valuable part of this staff since 2002, taking over after Dan Brown's long battle with cancer. Under Randy's leadership, we have had many outstanding secondary's including the 2005 group of James Sanders, Tyrone Culver and Richard Marshall, whom are all still actively playing in the NFL.
"Randy will remain an assistant until his contract terminates, helping in recruiting and film breakdown of upcoming opponents.
"Starting today, Tim Skipper will take over the duties of defensive coordinator. I feel Tim is one of the truly fine and upcoming coaches in our game.
"Not only was he an outstanding all-conference player in 1997-99, he has coached at a very high level with his position players while coaching running backs and linebackers in his tenure here as an assistant coach.
"I want to thank Randy for his 10 years of service and loyalty to Bulldog football and we wish him well in the future."