The European Court of Human Rights says a French court was wrong to convict Paris-Match magazine for revealing that Monaco's Prince Albert II had a love child in 2005.
The public interest outweighed privacy arguments in the case, the ECHR ruled.
After the Paris-Match article appeared, in May 2005, the prince sued the magazine, but later acknowledged that the child was his.
The boy was born to Nicole Coste, a French-Togolese flight attendant.
When Prince Albert, the reigning monarch in Monaco, learnt that Paris-Match was going to publish the article he told its owner, publisher Hachette Filipacchi Associes, not to do so. But the magazine went ahead, including photos of the prince with the child. The revelation also appeared in Bunte magazine in Germany.
In France the prince was awarded 50,000 euros (£40,000; $68,000) in damages, but in Germany his court action against Bunte was dismissed.
In its ruling, the ECHR found that the French court in Nanterre was wrong to convict Paris-Match over the article. It breached the magazine's freedom of expression, the ECHR said.
The ruling said the French court had erred in failing to distinguish between "information which formed part of a debate of general interest and that which merely reported details of the private life of the Prince of Monaco".
The child could not succeed to the throne, under Monaco's constitution, but the court found that "the public had a legitimate interest in knowing of the child's existence and being able to conduct a debate on the possible implications for political life in the Principality of Monaco".
"The court concluded that, in disclosing the information, [Nicole Coste] had sought to secure public recognition of her son's status and of the fact that the prince was his father, which were crucial factors in ending the secrecy surrounding him."
Back in 2005 Ms Coste was quoted as saying she met the prince on a flight in 1997, leading to a relationship and the birth of the boy, Alexandre, in August 2003.
Prince Albert has two children from previous relationships - 22-year-old daughter Jazmin and 10-year-old son Alexandre.
Last month he and his South African wife Princess Charlene announced they were expecting their first child.