The jury in Michael Gargiulo's murder trial returned its verdict Thursday ... finding him guilty of killing two women with a knife in California, including Ashton's friend, Ashley Ellerin. Jury deliberations began Monday.

Gargiulo will move onto the sanity phase of the trial, which will determine if he was sane during the killings, and this will begin on Tuesday.

As we reported ... Ashton took the stand back in May, going in front of the jury on behalf of his slain friend and providing crucial testimony.

Ashton said Ashley was supposed to be his date to a Grammys after-party in 2001, but she never showed.

The actor testified that on the night of the Grammys, he went to Ashley's home around 10:45 PM and knocked on the door, but there was no answer. Ashton says he peered through a side window and saw what he believed was red wine spilled on the floor, so he bounced.

Ashley was found dead in her house the next morning, lying outside the bathroom with more than 47 stab wounds ... and authorities said Gargiulo was the killer.

The judge and jury went on an emotional field trip back in June, checking out the place where Ashley died a gruesome death.