UBS said the employee was detained under a material witness warrant but declined to identify the person.
"Our understanding is that the respective employee, who has not been charged with any wrongdoing by the U.S. government, will remain in the United States pending discussions with the U.S. authorities regarding resolution of his status as a witness," a spokesman for UBS said in an emailed statement.
The U.S. Department of Justice and the Securities and Exchange Commission are investigating UBS's conduct in relation to cross-border services provided by UBS client advisors to U.S. clients from 2000 to 2007, UBS said
The DOJ is examining whether U.S. clients tried to evade U.S. tax obligations with the help of UBS client advisors, the bank said.
UBS said it was cooperating with the investigations.
The FT reported that Martin Liechti, the Zurich-based head of North and South America for UBS's international wealth management business, was detained as a material witness while visiting Miami late last month.