Weiner, the former congressman now running for New York City mayor, said the messages were "nothing new" and were part and parcel of the many issues he's spent the past two years working through with his wife, Huma Abedin, a longtime Hillary Clinton adviser.
His 22-year-old accuser says their correspondence took place only a year ago, while Weiner, 48, was in the midst of his exile from the public eye, ostensibly as he made amends to Abedin.
The exchanges, published on the gossip website TheDirty.com, consist largely of raunchy banter about an array of sexual acts. At one point, the man reported to be Weiner wrote to his online paramour: "I'm deeply flawed."
At a hastily arranged news conference a few hours after explicit texts and photographs emerged, a nervous-looking Abedin — usually highly protective of her privacy — took to the podium to defend her husband, who used the pseudonym Carlos Danger in some of the new exchanges.
"Anthony's made some horrible mistakes, both before he resigned from Congress and after, but I do strongly believe that's between us and our marriage," she said, adding it had required hard work and a "lot of therapy" to get past those mistakes.
"We discussed all of this before Anthony decided to run for mayor. I love him, I have forgiven him, I believe in him and as we have said from the beginning, we are moving forward."
Weiner, for his part, largely read from a statement he'd made earlier in the day.
"As I have said in the past, these things that I did were wrong and hurtful to my wife and caused us to go through challenges in our marriage that extended past my resignation from Congress," he said.
"This behaviour is behind me. I've apologized to Huma and am grateful that she has worked through these issues with me and for her forgiveness."
He was peppered with questions from the packed news conference about the timing of his conduct, however, and why he'd never revealed until Tuesday that he had continued sending explicit online messages to a strange woman long after he'd resigned.
While Weiner said some of the fresh allegations were untrue, he didn't elaborate. One exchange showed Carlos Danger suggesting buying a Chicago condo where he could meet up with his Internet friend and have sex.
Weiner insisted his wife knew of last year's exchanges.
"As I was more and more honest with her, I told her everything," he said.
But in a New York Times profile of the couple published earlier this year, Abedin made several references to the scandal "two years ago," never hinting that her husband's conduct had continued.
The couple wed in 2010 in a ceremony officiated by former president Bill Clinton, himself no stranger to sex scandals.
In all, Tuesday's news conference had the surreal tone of a public therapy session — at best, it provided a painful glimpse of an ambitious but troubled soul and his wounded but perhaps equally ambitious wife; at worst, it was the final nail in the coffin of a once-promising political career and, perhaps, his marriage.
Weiner announced his resignation from Congress in June 2011 after spending days denying he'd sent lewd messages and photos of his bulging boxer briefs to Internet strangers. Abedin was pregnant at the time, and the pair have said publicly that they've spent the past two years working on their marriage and raising their young son.
Earlier this year, Weiner announced he was running for New York mayor. On the campaign trail, he has often apologized for his past misdeeds, asked New Yorkers to give him a second chance and spoken lovingly of his wife.
Amid the fresh allegations, Weiner said he was staying in the mayoralty race as calls for him to step aside mounted. Three of his rivals for mayor immediately called on Weiner to drop out.
Bill de Blasio and Sal Albanese, fellow Democratic candidates for mayor, urged him to exit the race. So did John Catsimatidis, a Republican candidate.
"Enough is enough," de Blasio said. "I'm calling on Anthony to withdraw from this race, for the good of the city that I know he loves as much as all of us."