The 51-year-old actress filed a report with the Los Angeles Police Department accusing Nick Abramovic of committing the offences during an appointment at his Bionic Method on August 7.
This happened after the pair were involved in a public spat over an unpaid bill of $US2700 ($A3900) for three sessions Kate attended over a two-week period in the summer.
View this post on Instagram A post shared by Kate Beckinsale (@katebeckinsale)
Abramovic strongly denies the claims and no charges have been filed, the Daily Mail has reported.
He told the New York Post: "I just want to be clear: I did not assault her at all. I categorically deny these allegations.
"In the five years since I've run Bionic, I have never had this type of complaint from any individual that I have treated.
"Over the years, I have treated hundreds of women without incident. I'm not trying to defame Kate at all.
"It's weird to me that this is happening. I'd like to think this is just a misunderstanding."
Abramovic describes his Bionic Method as a "massage that's more than just physical relief".
He adds it is designed to "release emotional and energetic blockages".
Beckinsale has referred to her alleged sexual assault on Instagram by posting an image on Instagram that said: "Boys will be held accountable for their f***ing actions."
She also tagged @survivorsofsexualassault in the upload.
Beckinsale also posted screen shots of Yelp reviews from previous clients of Abramovic's that claimed he "targeted" female clients and that "something felt immediately off and weird" when they visited his building.
Abramovic says Beckinsale never intended to pay him for her treatment, and said on his Instagram: 'Over a two-week period Kate Beckinsale scheduled $US2700 ($A3900) worth of sessions with me, promising to pay us later.
"But after her last session she completely ghosted us."
Nick added he was served with a legal notice informing him of the criminal allegations only after he hit out at Beckinsale on social media.
His lawyer George Shohet said: "We're looking into a defamation case as this appears to be a retaliatory act after Mr Abramovic demanded payment from Ms Beckinsale."
The actress' lawyer Marty Singer, responded: "I believe it is clear that the claim that my client fabricated a police report to avoid paying money is a malicious, false and defamatory statement.
"The belief that my client would file a false police report and wind up having to pay more money for my legal services than pay this person is the most ridiculous concept I've ever heard in my career as an attorney.
"(Nick) did not ever offer to record the session. That statement is not truthful."
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