Mel Gibson & Wife Robyn Gibson -- Officially Divorced

Mel Gibson and Robyn Gibson are officially single -- a judge signed the divorce decree moments ago.

The only big change -- Robyn is going back to her maiden name ... so from now on it's Robyn Moore.

As we reported, the couple's been negotiating a financial settlement for over a year ... and we're told, it's finally a done deal. There is no prenup.

The couple's reported net worth is in the $900 million range -- but sources tell us, it's actually less.

Mel was represented by Larry Ginsberg.