Mel Gibson Single by Sundown

12/23/2011 8:20 AM PST

Mel Gibson -- Single by Sundown

Mel Gibson will officially be back on the market by the end of the day ... because a judge is about to sign his divorce decree.

The couple and their lawyers have been negotiating a financial settlement for more than a year, and we're told the deal has finally been struck.

Now, the paperwork has been submitted, the judge will sign off and Mel will be a single guy by the end of the day.

There was no prenup -- remember, they were married for 28 years before officially separating.  Although there are reports the Gibsons' net worth approached $900 million, but we're told it's less.

The couple separated back in 2006 -- shortly after his infamous Malibu DUI arrest -- and Robyn filed for divorce in 2009 -- shortly after learning Mel's then GF Oksana Grigorieva was pregnant.

The divorce has been surprisingly civilized, with Robyn even writing a letter of support for Mel last year ... in his legal war with Oksana Grigorieva.  Robyn wrote in all the years she was with Mel, he never struck anyone in their family.