UFC -- We're Investigating BJ Penn ... Over Sexual Assault Allegations

The UFC has launched an investigation into legendary fighter BJ Penn -- after a former writer on Penn's website claimed BJ sexually assaulted his girlfriend.

The writer is Pedro Alex Carrasco -- who went to Twitter and said BJ recently came to his home "when I wasn't there and forced himself on my girl, pulled her clothes off, then blamed me, said I looked at his GF ass."

Pedro's girlfriend reportedly filed charged is Ohio -- and the UFC has already announced it's looking into the situation.

"UFC is aware of the recent allegations made against BJ Penn."

"Every athlete is deserving of proper review and this situation, as with any serious allegation, will be investigated by an independent party and thoroughly reviewed by the UFC."

"The UFC organization will also cooperate with any law enforcement investigation."

UFC says they've put any plans to book Penn in a fight on hold until they know more about the situation.