Indianapolis-based USA Gymnastics said via Twitter that it was in the process of naming an interim board after receiving the resignations of 18 board members.
Three other board members resigned last week as Larry Nassar, the former Michigan State University and USA Gymnastics physician, faced more than 150 women in court over his abuse of two decades. He was sentenced to up to 175 years in prison
for sexually abusing patients under the guise of medical treatment.
The Nassar scandal has led to a widening inquiry into how several institutions failed to stop the abuse for so long.
USA Gymnastics, the US Olympic Committee and Michigan State have denied wrongdoing. USA Gymnastics has said it reported the sexual abuse allegations to authorities when it learned about the abuse.
Blackmun also said he would yank the organization's status as the country's gymnastics regulator unless the board met the deadline and named an interim board by February 28.
The Olympic Committee has called for an investigation by an "independent third party to examine how an abuse of this proportion could have gone undetected for so long."
USA Gymnastics, which was already undergoing an overhaul due to the scandal, thanked board members for their service.
"We are grateful for the time and effort each has devoted to USA Gymnastics," it said on Twitter.
It had asked former federal prosecutor Deborah Daniels in late 2016 to review its policies on handling sexual misconduct.
In June, Daniels released a report highlighting numerous shortcomings. She made 70 recommendations, including requiring members to report suspected sexual misconduct to legal authorities and the US Center for SafeSport, and implementing an abuse prevention training plan for members, parents and athletes.
"USA Gymnastics embraces not only the changes necessary as called for by the USOC and the Deborah Daniels report, but we also will hold the organization to the highest standards of care and safety in further developing a culture of empowerment for our athletes and members," the governing body said Wednesday on Twitter.