Russell Brand Denies Alleged Sexual Assault on 'Arthur' Movie Set

Russell Brand is vehemently denying accusations he got drunk on a movie set and sexually assaulted an actress ... he says there's no way her claims are true, because he was sober.

In new legal docs, obtained by TMZ, Russell pushes back on a lawsuit alleging he exposed his penis to a woman on the set of the movie "Arthur," and then sexually assaulted her in a bathroom.

Russell claims he doesn't recall the actress who is suing him over the alleged 2010 incident, and he says her entire suit rests on the false claim he was intoxicated on set.

RB says he was just playing a drunk in the movie ... and he hasn't done drugs or had a sip of alcohol since Dec. 13, 2002 ... claiming he was 8 years sober at the time of the alleged assault.

In the docs, filed by his attorney Andrew Brettler, Russell says the woman "is unable to distinguish acting from reality." He says his job was to "act drunk and portray a drunk," but "he did not drink a sip of alcohol or consume any drugs at any time during production."

As we reported ... Russell's accuser, who is suing as a Jane Doe, claims prior to the alleged assault he "appeared intoxicated, smelled of alcohol, and was carrying a bottle of vodka on set."

Russell says the vodka bottle was just a prop for the film ... and contained no real booze.

The woman claims Russell followed her into a bathroom where he sexually assaulted her ... but he says that's not true either, claiming her memory is "faulty."