A judge in Zurich did so even though the leaked documents referred to accounts in the Cayman Islands.
Judge Sebastian Aeppli fined Rudolf Elmer, 55, more than 6,000 Swiss francs ($6,250; £4,000). However he rejected prosecution demands to give Elmer an eight-month prison sentence.
Elmer also said that he had handed confidential Julius Baer banking files to tax authorities, and later the Wikileaks website run by Julian Assange, because he had wanted to expose tax evasion by businessmen and politicians.
Mr Assange has said that he will publish that information within weeks, once it has been checked.
Speaking minutes before he returned to court to hear the verdict, Elmer told reporters: "I made big mistakes, I admit that... I wouldn't say it was revenge [against Baer], but I defended myself. That's human nature."
Elmer was subsequently arrested after a court verdict in a separate case. Chief prosecutor Peter Pellegrini said: ‘He is in police detention and will be questioned today.’
Mr Pellegrini confirmed the new investigation centres on Mr Elmer’s visit to London on Monday.