The woman, who remains anonymous, said the incident occurred in 2008 when she was working in the same building as the BBC in Los Angeles.
Minutes later, Brand was recorded laughing about the encounter, with his co-presenter saying that the comedian “showed his willy to a lady”.
Brand has not yet responded to the latest allegation. His co-presenter Matt Morgan told the BBC he was “not aware until now of the nature of this encounter”.
Describing the incident, the woman alleges that she answered the door to Brand and his team, who were there to pre-record an episode of The Russell Brand Show for Radio 2.
She then went to the bathroom to get some sinus medication, walking past the radio studio. While squatting to look through the medicine cabinet, she said she felt someone behind her.
As she turned around, she claimed she was faced with a man’s crotch. “I was startled and got up and I realised it was the man that I’d let in – Russell.”
The woman claimed that Brand said: “Oh, I think you’re a bit alright. I think you’re a bit of alright.”
She said he told her he was going to call her Betty. When she said that wasn’t her name, he allegedly replied: “Well, I’m gonna fuck you.”
“And I said, ‘No, you’re not,’” she added.
The woman claimed that Brand then pulled his penis out on his hand and “pretty much served it to me as you would be serving someone some food”. She added that the bathroom door was closed and that she felt trapped.
“There was a bit of banter going on because I didn’t know what to do,” she added. The woman claimed that Brand then put his penis back in his trousers, and after hearing one of his team members bang on the door, the comedian left the bathroom.
The woman said she returned to her desk in disbelief and texted a BBC employee in the office about it. The employee told her he already knew about the incident because Brand was joking about it in the studio.
You can listen to Brand and Morgan’s discussion about the encounter here.
News of this new allegation comes after the investigation by The Sunday Times, The Times and Channel 4’s Dispatches programme into Brand’s alleged behaviour, with multiple women coming forward.
The comedian has been accused of rape, sexual assault and emotional abuse. He has strongly denied the claims.