The DNA test revealed in January this year that 86-year-old Albert II is the biological father of Boel, putting an end to her long-running paternity claim.
Albert, who abdicated six years ago in favour of his son Philippe, had long contested Boel's claim.
As well as being recognised as a princess, Boel (52) will change her surname to her father's, Saxe-Cobourg.
Her two children Josephine and Oscar will also receive royal titles and the right to be referred to as his or her royal highness.
"A legal victory will never replace the love of a father, but does offer a sense of justice, further reinforced by the fact that many children who have gone through the same ordeal will find strength to face them there," Boel's lawyers said.
The court's sudden ruling came as a surprise. Its decision had been scheduled for October 29.
