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.