The Federal Administrative Court said Switzerland's tax office could not hand over the data as the request was based solely on a suspicion of tax evasion.
US tax officials have, for years, been investigating US citizens that they suspect are using Swiss bank accounts to avoid paying tax.
These probes have caused political tension between Switzerland and the US.
The Swiss government is hoping that new legislation, under which Swiss authorities would provide more help in cases of tax evasion, will help end the dispute.
But the US has yet to approve that compromise.
In the meantime, US tax authorities are investigating 11 Swiss banks including Credit Suisse and Julius Baer.
In 2009, UBS, under pressure from the US, released the names of 4,500 clients and paid a fine.