Marriage was NOT as good as it gets for Cuba Gooding Jr.'s wife.

She just filed for separation from the Oscar-winning actor after 20 years of marriage, TMZ has learned.

Sara Kapfer filed today in L.A. County Superior Court, citing irreconcilable differences. She and Cuba were high school sweethearts and got married in 1994. They have 3 children, 2 of whom are minors.

Sara wants joint legal and physical custody. According to the docs, child support will be determined later.

It's surprising, just last year, Cuba gushed about his wife on "The View," talking about how wonderful it is to be married to someone for so long.

Source: TMZ