Matthew Lush said fans seeking out his channel were now being redirected to the beauty brand instead.
The company told the BBC it had not requested the change but would not say if it would give the address back.
Google said it was "sympathetic" to Mr Lush's situation and that the decision was made by an algorithm.
Mr Lush, who has 10 times as many YouTube subscribers as the soap brand, told the BBC he felt "betrayed".
Mr Lush registered the username Lush in 2005, which was attached to the web address youtube.com/Lush.
Last year, Google started allowing YouTube creators to customise their channel addresses.
Google said its algorithm decided which address Lush Cosmetics was given, based on data from YouTube, Google+, its search engine and other sources.
Mr Lush told the BBC: "I've been doing YouTube for a long time, but they've stripped this from me."
"I have that address on thousands of bracelets that I've sold, and it's embedded in my videos. I can't remake those now," he said.
Google said Mr Lush was a "valued creator" and that it had offered to pay for new marketing materials for his channel.
The BBC understands it would be at the discretion of Lush Cosmetics to allow Mr Lush to use the address again.
Google told the BBC: "We continue to make every effort to work with creators to support their needs on YouTube."
Mr Lush's fans have set up a petition to ask the cosmetics company to hand over the web address.
One fan even posted pictures of herself burning Lush products on Twitter.
Like many YouTube stars, Mr Lush earns a living from advertisements displayed around his content.
He said: "My brand is very important to me too. A brand is how you grow."
A popular video blogger has hit out at cosmetics brand Lush after he lost control of a YouTube address he had been using since 2005.