"To be clear, the picture was of me and I sent it," he said, ending 10 days of vague answers to questions about the bulge.
Weiner, 46, also admitted that he shared lewd pictures and lurid banter with at least six women on the Internet and over the phone in the past.
Several of them began speaking out last night, including Lisa Weiss, a 40-year-old blackjack dealer from Las Vegas.
"I gave him my number and he called me from his office, and we proceeded to talk dirty for at least 30 minutes," Weiss claimed to RadarOnline, describing a steamy chat that occurred after the two traded text messages.
Weiner apologized profusely for the harm he caused to his family and friends, but he also made it clear that he wasn't planning to resign.
"I don't believe that I did anything here that violates any law or that violates my oath to my constituents," Weiner said.
But it's unclear if Weiner - who political observers believe was going to one day run for mayor of New York City - will ultimately end up having to look for a new job.
House Minority Leader Nancy Pelosi last night called for an ethics investigation to see if Weiner broke any House rules.
Weiner's scandals echoed in Philadelphia, where a number of local politicians rely on Twitter to keep constituents informed of the latest bills they've introduced or meetings they've attended.
Much of it is boring, to be sure, but local social-media experts say that that's exactly the line elected officials should toe.
"Basically, everything in social media - assume its public," said Andrew Mendelson, chairman of the journalism department at Temple University.
"Some people have been tweeting, 'He wouldn't flash himself; why is it OK to do it on Twitter?' It's not."
Mendelson added that people shouldn't rush to point out dangers lurking in the ever-evolving world of social media, but should look instead toward monitoring questionable behavior in general.
Weiner apparently thought that he sent the photo as a direct message, but ended up tweeting it publicly instead. Regardless, Mendelson said that it's a lame excuse for a politician's bad judgment call.
U.S. Rep. Bob Brady relies on a staffer to update his Twitter account, said spokeswoman Karen Warrington. "He just tweets his votes and his positions on legislation," Warrington said, "but nothing personal."
Like Brady, Rep. Chaka Fattah uses his account mainly to put out 140-character press releases. Mayor Nutter and some on City Council use their accounts to talk policy and chat with voters. A tweet from Nutter yesterday read: "Heading to @NBC #EducationNation at the Constitution Center to highlight the successes we've achieved and the challenges we still face."
Nothing sexy there, but nothing Nutter would have to hold an awkward news conference over, either.
"In the world of social media, you probably shouldn't put something out in the universe that you wouldn't want to read in the paper the next day - or online in the next five minutes," said Desiree Peterkin-Bell, director of Nutter's office of communications and strategic partnerships.
The rule is ridiculously simple and intuitive - and that apparently creates problems on its own. "Sometimes, easy rules are the hardest to follow," Peterkin-Bell said.