According to excerpts released by the network Thursday, Phelps replied: "It was a bad mistake. I mean, we all know what, you know, what you and I are talking about. It's a stupid mistake. You know, bad judgment. And it's something that, you know, I have to, and I want to teach other people not to make that mistake."
The interview was to air Friday morning on "Today" and Sunday night on "Dateline."
USA Swimming suspended the Olympic great for three months after the photo was published in a British tabloid Feb. 1. He also lost his sponsorship from Kellogg.
Phelps has previously apologized for his poor judgment.
The photo was taken at a house party while Phelps was visiting Columbia, S.C., in November during an extended break from training after he won a record eight gold medals in Beijing.
"There was probably two or three people there I didn't know," he said. "It was a very small group. Six or seven people probably total in the whole house. Like nothing major. You know, not like a giant college house party. It was nothing like that. It was just a small group and we were just sitting around and celebrating."
Phelps said he trusted his friends that the people he didn't know there could also be trusted.
"I'll say that there are a lot of people out there who want to take advantage of any situation they have. ... Sometimes you learn the hard way," he said.
Phelps said he became aware of the photo a day or two before it was published.
"It's not about money to me," he said of the fallout. "So, you know, the contract side of things, yeah, I was disappointed. But, you know, I think the biggest thing is who I hurt the most. Like, if I lost money, OK. It's not an issue with me."