RCS launched arbitration proceedings in late 2018 to nullify the 2013 sale of its historic headquarters in central Milan to Blackstone, saying the U.S. investment firm had paid too low a price at a time when RCS faced financial difficulties.
The Milan tribunal rejected all claims advanced by RCS on Friday, a court document seen by Reuters showed.
Shares in RCS Mediagroup were down 3.4% and Cairo Communication stocks fell 7.2% by 0900 GMT, compared with a broadly flat market.
A Milan-based analyst said the shares fell because of concerns that RCS now faced financial risks as a result of two U.S. lawsuits filed by Blackstone that had been put on hold pending the Italian arbitration ruling.
Blackstone declined to comment. RCS referred to comments it made on Friday after the arbitral award, saying the ruling confirmed the group did not act "unfairly or recklessly".
It had said on Friday that this "strengthens the position of the company before the Supreme Court of the State of New York".
The U.S. investment firm previously filed two lawsuits in New York accusing RCS of falsely claiming it still owned the building and of improperly blocking its sale to Germany's Allianz. RCS denies any wrongdoing.
A source close to Blackstone said it was seeking up to $600 million in damages from the Italian publisher and from its chairman and main shareholder Urbano Cairo, who took control of the company in 2016.
RCS' financial report, published on its website, shows it has not set aside risk provisions for the legal dispute.
Italian broker Banca Akros wrote in a report on Monday it was "very sceptical of the actual chances for Blackstone in winning a large compensation".
(Reporting by Claudia Cristoferi, Elvira Pollina, editing by Kirsten Donovan)