49ers Star Ahmad Brooks -- Accused In Sexual Assault Lawsuit

San Francisco 49ers linebacker Ahmad Brooks has been accused of sexually assaulting a passed out woman at a then-teammate's home ... according to a lawsuit obtained by TMZ Sports.

The woman making the allegations is the same woman who previously claimed she was raped by Ray McDonald ... a former 49ers defensive end who now plays for the Chicago Bears.

McDonald had sued the woman for defamation ... after the San Jose PD investigated the allegations but dropped the case due to insufficient evidence.

Now, the woman is firing back in the defamation lawsuit -- claiming she believes there is SURVEILLANCE VIDEO from the night in question ... which shows part of the alleged sexual assault.

According to the docs obtained by TMZ Sports, the woman says on December 14th, 2014 she was at McDonald's home and slipped and fell on the deck around his swimming pool, knocking herself unconscious for more than 8 minutes.

While she was blacked out, the woman claims the video will show McDonald's friend Brooks "groped her person in a sexual manner."

She claims McDonald then carried her up to his bedroom where he had sex with her ... without her consent.

The woman -- who hired Gloria Allred as her attorney in the matter -- is going after McDonald for assault and battery, false imprisonment and infliction of emotional distress. Ditto for Brooks. She and Allred just held a news conference to talk about the case.

We've reached out to Brooks for comment -- so far, no word back.