Miriam Lee filed paternity and child support docs in Orange County, FL last year alleging McKnight is the father of her son.
Brian never filed docs addressing her claim .. so the judge has ruled Brian's the daddy.
And now for the bad news -- the judge also decided Brian must fork over 14 years worth of child support ... which comes to a whopping $341,640.
We're told Brian's monthly support payment is $11,388 a month.
A rep for Brian McKnight tells us, "The judgment in Florida was a default judgment made without Brian present. He's still looking to establish paternity, and is awaiting the results of a recent test."