Diane Lane is the one who threw in the towel ... we've learned she's the one who filed legal docs for divorce from Josh Brolin.
We found the legal docs, which were filed February 15 -- right after Valentine's Day -- in L.A. County Superior Court.
We know Lane claims she has been separated from Brolin for quite a while.
As for why they split ... they had lots of issues. Four months after they married in 2004, Brolin was arrested for spousal battery, but the charges were dropped.
He was also arrested for fighting in a bar in Louisiana in 2008. Drinking was involved. Interestingly, when he was arrested, he turned to his buddy, actor Jeffrey Wright, and said, "My wife is gonna be so happy." Brolin eventually copped a plea.
And Brolin was arrested in Santa Monica this past New Year's Day for public intoxication.