|English: The old logo of Credit Suisse. (Photo credit: Wikipedia)|
Douglas Hornung, a Geneva-based lawyer acting for the former Credit Suisse employee, said the ruling was made on June 21, confirming a preliminary decision in January.
The judgment could render it more difficult for banks to reach individual settlements with U.S. authorities in a long-standing row over tax evasion.
Credit Suisse spokesman Marc Dosch declined to comment.
The court ruling comes only days after Swiss lawmakers threw out a draft law aimed at providing a legal basis for banks to hand over this kind of data to U.S. authorities in an attempt to avoid prosecution.
The government plans an executive order to allow banks to hand over data but its efforts could be stymied by more legal action by bank staff fearful of U.S. extradition if they leave the country.
"It will set a precedent and could be repeated for other employees who had access to U.S. clients," Hornung, who also represents other former bankers, told Reuters on Tuesday.
Credit Suisse, like other Swiss banks subject to U.S. investigations, has already made several transfers of data on employees linked to accounts of its U.S. customers in an attempt to avoid indictment and minimize fines.
The last transfer was in June.
Switzerland's biggest bank, UBS, was forced to pay a $780 million fine in 2009 and deliver the names of more than 4,000 clients to avoid indictment.