The married lawmaker also admitted to having inappropriate contact with several women online, including one who may be a former adult actress. An ethics committee investigation into his behavior has been launched.
He took to the podium in Manhattan on Monday to admit he sent several pictures of himself online, including the infamous underwear shot, and a bare-chested photo he reportedly sent to a woman on Facebook.
Weiner admitted to carrying-on with six women online over the last three years, but said he never met them in person.
"To be clear, the picture was of me and I sent it," Weiner said. "I am deeply sorry for the pain this has caused my wife and our family and my constituents, my friends, supporters and staff."
At the news conference, Weiner also talked about his wife, Huma Abedin, and called her a remarkable woman. He apologized publicly to Abedin, holding back tears at a packed press conference in Manhattan.
Weiner said he is not planning on separating from his wife, who is a longtime aide to Secretary of State Hillary Clinton. Whether or not she feels the same is unclear. She has yet to comment and was not at the news conference.
But one woman who is talking is 36-year-old Meagan Broussaard, who says she frequently exchanged online messages with the congressman.
"He would say just 'Good morning, how are you doing today?'" Broussaard said.. "'What are you wearing? What do you like?' You know, in the bedroom, that sort of thing." She went on to say that she doesn't think Weiner is a bad guy but does have "issues."
Weiner's colleagues on Capitol Hill aren't exactly rallying around him. Some said voters will decide if they can forgive his indiscretions.
"I know Congressman Weiner and I wish there was some way I can defend him, but I can't," Sen. Harry Reid said.
Republicans are calling on Weiner to resign.
Conservative blogger Andrew Breitbart, who broke the story, says he has an X-rated picture of the congressman that he's threatening to release if Weiner retaliates.