The website said Sharpton was recruited by a police-FBI task force to record conversations with Mafia figures using electronic equipment hidden in a briefcase. It said Sharpton was known as Confidential Informant No. 7 in court papers.
Sharpton said he went to the FBI after he was threatened by mobsters working in the music business, a story he recounted in his 1996 book, "Go and Tell Pharaoh."
He read a passage from the book Tuesday and said, "Nothing new about that story."
Sharpton acknowledged using a recording device in a briefcase but said he did nothing wrong.
"In this situation I did what was right," he said.
The Smoking Gun posted dozens of pages of documents that it said showed Sharpton's dealings with mob figures. Sharpton said his lawyers would review the documents.
Sharpton said he was not an associate of mobsters but was threatened by mob figures who controlled the music business.
He said he went to the FBI and said, "I'm being threatened. These are the kinds of guys you should be going after."