Rep. Christopher Lee, a married two-term Republican lawmaker, said he was quitting because he regretted actions that have hurt his family and others.
The gossip website posted a photo of Lee with his shirt off and apparently flexing before a mirror.
In an e-mail, Lee said that his resignation was effective immediately. The statement offered no confirmation or details of a Craigslist posting.
An anonymous woman, described only as a Maryland resident, 34 years old and government employee, provided the website with e-mails she said were an exchange between her and Lee in response to an ad she placed in the "Women Seeking Men" section of Craigslist.
Lee allegedly identified himself as a divorced lobbyist, but the woman became suspicious that he had misrepresented himself.
"I regret the harm that my actions have caused my family, my staff and my constituents," Lee said in his resignation e-mail. "I deeply and sincerely apologize to them all. I have made profound mistakes and I promise to work as hard as I can to seek their forgiveness.
"The challenges we face in Western New York and across the country are too serious for me to allow this distraction to continue, so I am announcing that I have resigned my seat in Congress effective immediately."
The Associated Press contributed to this report.