Weil, who was an executive at UBS's global wealth management business, jumped for joy and kissed his wife at the Florida court after jurors returned the not-guilty verdict on Monday in less than two hours.
The banker, who had been extradited to the US to stand trial, had faced up to five years in prison and a hefty fine if found guilty.
Prosecutors at the Fort Lauderdale trial, which started on October 14, had tried to convince jurors that the 54-year-old Swiss citizen helped US clients conceal $20 billion in assets from tax authorities between 2002 and 2007.
Weil, in his role based in Switzerland, had supervised overseas activities that served some 20,000 customers.
His attorney Matthew Menchel said his client was not aware of any irregularities committed by subordinates.
"There is no document to prove" any wrongdoing, Menchel said during closing arguments.
After the verdict, Menchel told reporters: "This is a case that should never have been brought."
Fired by UBS in April 2009, Weil joined the wealth management firm Reuss Private Group in 2010 as a consultant, then became head of the company in early 2013.
That career came to an end when he was arrested in October 2013 after using his real name to check into a luxury hotel in Bologna, Italy and he was extradited to the United States.
Testimony in exchange for immunity
During the trial, Weil seemed relaxed and he smiled often.
Prosecutors called several witnesses, mostly former UBS bankers who collaborated with the US government in exchange for immunity.
Menchel had said in court that such witnesses had merely tried to save themselves by incriminating his client.
"Who . . . should be punished instead of getting sweetheart deals?" Menchel had asked.
Menchel had criticized Martin Liechti, the former head of UBS operations in America, whose detailed testimony formed the backbone of the case.
During his five days of testimony, Liechti had portrayed Weil as a greedy character who prioritized profits ahead of the law.
During closing arguments, prosecutor Jason Poole had said "this is a simple case."
"Mr Weil as early as 2001 was aware of what this business was about, and it was something he participated in."
UBS has been cooperating with US authorities under an agreement reached in February 2009 that resulted in a $780 million fine.
The bank has turned over the names of many of its US clients suspected of tax fraud.
Weil has been living in New Jersey under the supervision of authorities after being freed on bail of $10.5 million last December.
He had been ordered to surrender his passport and wear a GPS tracking monitor.
The Weil case was a high-profile part of the massive US crackdown on alleged offshore tax dodging, particularly in Switzerland, renowned for its banking secrecy.