He revealed her identity as Julie Swetnick on Wednesday.
In addition to writing that she "observed Brett Kavanaugh drink excessively at these parties and enage in abusive and physically aggressive behavior towards girls", she mentions having seen him at a string of parties known as "Beach Week".
Avenatti also told reporters the woman, whom he did not name, has multiple security clearances and will "literally risk her life" by coming forward.
Judiciary Committee Republicans say they are now reviewing Swetnick's allegations.
The New Yorker published new allegations against Kavanaugh dating to the 1983-1984 school year, when he was a freshman in college.
Kavanaugh has repeatedly denied ever sexually assaulting anyone. Avenatti would not elaborate on the number of clients but said he represents them alone.
"This morning Michael Avenatti provided a declaration to the Judiciary Committee", spokesman Taylor Foy said. She then said that Kavanaugh pinned her to the bed and groped her over her clothes, and attempted to take them off. Ford alleges that when she tried to scream, Kavanaugh put his hand over her mouth.
Kavanaugh and the White House have not yet responded to these latest allegations. Note, she didn't accuse Mark or Brett of raping her, but said that they were present during the alleged incident.
FOX 5 has not been able to corroborate Swetnick's allegations and is investigating the claims.
"I have a firm recollection of seeing boys lined up outside rooms at many of these parties waiting for their "turn" with a girl inside the room", she said.
A spokesperson for the Time's Up movement later issued a statement calling for Kavanaugh to withdraw: "His confirmation would compromise the integrity of the court for generations to come", he said.
Senate Democratic leader Chuck Schumer called on McConnell to apologize to Ford "for labeling her allegations a 'smear job, '" adding, "I challenge you, Leader McConnell, if you are so convinced this is a smear campaign, you'll have no problem with an FBI investigation to prove your case".