67-year-old Swann took the job back in April 2016 -- replacing longtime AD Pat Haden.

No word as to why Swann is resigning.

USC President Carol Folt sent an email saying, "Lynn has been a leader on and off the field at USC for nearly five decades, and he will forever be a valued member of the Trojan family."

"Lynn has led our Athletics Department to five national championships in five different women’s and men’s sports during his tenure," Folt added. "Our student-athletes’ cumulative GPA for the past two years and our graduation rate of 86% last year were all-time highs for USC Athletics."

"We appreciate Lynn’s leadership of the department over these past three years and will continue to be grateful for his historic contributions to USC."

Swann has been under fire in his role as AD basically since he took the job. Most notably, he was heavily criticized for the infamous admissions scandal at USC where students were deceitfully using the Athletic Dept. to get into the school.

Remember, it was Lori Loughlin's daughter, Olivia Jade, who falsely claimed to be a rower to gain admission to the school as a member of the USC crew team.

Swann was never formally accused of wrongdoing -- but the whole thing was a bad look for his athletic dept.

USC says Dave Roberts, currently serving as special adviser to the President, will take over as interim AD. He's also the Vice-Chair for the NCAA's Committee on Infractions. He's reportedly been at USC since 2010.

Swann was a star WR at USC back in the '70s. He's a College Football Hall of Famer and won a championship with USC back in 1972.

He went on to be a superstar in the NFL, winning 4 Super Bowls with the Pittsburgh Steelers.