Paula Patton Files for Divorce from Robin Thicke

Robin Thicke tried and tried hard to save his marriage, but it didn't work. TMZ has learned Paula Patton has filed for divorce ... and as for the most important asset in the marriage ... she wants joint custody.

Paula cites the usual ... irreconcilable differences.

She wants joint legal and physical custody of their 4-year-old son Julian.

The divorce was filed in L.A., so there will be no issues over whether Robin may have strayed. It's irrelevant.

We're also told the divorce is "harmonious," and there are no real issues that will cause a conflict.

Paula says the date of separation was February 21.