Gary Dennis, a nursing home assistant from Pennsylvania, said he was clearing out a box of old tapes in his home and found one labelled with Kelly's name.
Mr Dennis said: "To my shock and surprise, R. Kelly appeared to be on the tape, but not in concert... instead he was sexually abusing under-age African-American girls.
"He was telling them what to do and what to say and it appeared he was in control of the camera"
Mr Dennis said he did not know how the tape came to be in his possession, but that it may have come from a friend.
Gloria Allred, the lawyer representing alleged victims in an abuse case against Kelly, said the tape appeared to show a separate incident from the 10 counts of abuse the singer is already facing.
However, she admitted she could not be "100% certain" that the man in the tape is R Kelly.
Ms Allred said the tape had been given to police.
Steve Greenberg, a lawyer for Kelly, said that "The doubt here is self-evident, with reporting that the man on the tape kinda, sorta looks like R. Kelly... that doesn't make it him. It is not him."
"It is obviously now just open season on R. Kelly", Mr Greenberg added.
The RnB star denies 10 counts of aggravated sexual abuse, after being taken into custody in Chicago last month.
Last week, Kelly used a TV interview to plead with viewers that he never had sex with anyone under the age of 17 and had never held anyone against their will.
The singer was cleared of child pornography charges in 2008.