"While separated from my second wife, prior to the divorce, I had sexual relationships with other mature adult women," Barton, 68, admitted in a written statement.
"Each was consensual. Those relationships have ended. I am sorry I did not use better judgment during those days. I am sorry that I let my constituents down," the statement added.
Barton claimed in a statement to ABC News that one of the women "threatened" to make his private photos and messages public and that he told her he could take the issue to the Capitol Police. He also said late Wednesday that he accepted an offer from Capitol Police to launch an investigation into the photo's release. The Washington Post was the first to report Barton threatened to go to Capitol Police if she released the private photos.
"This woman admitted that we had a consensual relationship," he said in a follow-up statement late Wednesday. "When I ended that relationship, she threatened to publicly share my private photographs and intimate correspondence in retaliation."
Barton is not expected to immediately resign, according to a spokeswoman.
The episode is seen as an embarrassing disclosure for a prominent House Republican — the longest-serving congressman from Texas — and one that could threaten his political career.
Barton, a former chairman of the influential House Energy and Commerce Committee, recently announced plans to run for re-election.
He has been a vocal advocate against sexual predators online as a member of a congressional task force and spoke at a press conference on Capitol Hill alongside victims in 2007.
ABC News' Arlette Saenz, MaryAlice Parks and Devin Dwyer contributed to this report.