The country's High Court made the ruling today.
The singer already has two children from the African country - David, adopted in 2006, and Mercy, adopted in 2009.
Malawian judiciary spokesman Mlenga Mvula said the singer was inside the court in Lilongwe when the ruling was given.
"In fact, at the time we were granting her the permission she was in the courtroom with her lawyers," Mr Mvula said.
But less than two weeks ago Madonna denied reports she'd applied to adopt any more children, during a charity visit to Malawi.