The former King fought the claims for seven years and even failed a DNA paternity test in October 2018 after being ordered to do so by the Brussels Court of Appeals.
"I unreservedly endorse the communique that the King and Princess Delphine have published and I join in the spirit of this message", he said.
Delphine de Saxe-Cobourg (yes, the same name as the British royal family's) has visited her half-brother, Belgium's King Philippe, for the first time.
Recently known as the Delphine Bowl, Saxe-Coberg filed a lawsuit in 2013 seeking to have the former king recognized as his father.
The test confirmed he was indeed her biological father, which he had refused to admit since the revelation in 1999 of an old extra-marital affair.
The "warm" meeting between Delphine and the king took place at the monarch's Palace of Laeken official residence, the royal family said. We had the opportunity to get to know each other during a long and rich exchange which allowed us to talk about our respective lives and our common interests.
Rumours about Albert and Princess Delphine's mother, the aristocratic wife of a wealthy industrialist, had been around for years but Albert's two children never met each other. "This bond will further develop within the family setting".
She was born from King Albert II's 18-year-long affair with Baroness Sybille de Selys Longchamps.
Albert II married Paula Paolo Rafo de Calabria, Queen of Belgium in 1959, and became king in 1993. The king's extramarital relationship continued until 1984. After a DNA test showed that the 86-year-old is, a court granted her the title of princess earlier this month.
In a statement signed simply "Albert", the current monarch's father on Friday backed the move.
His dismissive rejection of the hard-fought recognition was a step too far and she made a decision to press for legal recognition as a royal.