Zurich prosecutors say Rudolf Elmer stole client data after being sacked by his employer and then tried to extort money from Bank Julius Baer and senior executives.
A prosecution statement obtained by the AP ahead of the trial also alleges Elmer illegally gave details on the banks offshore clients to tax authorities and media.
Prosecutors are calling for an eight-month suspended sentence and fine for what Julius Baer says was a vendetta by a disgruntled employee against it.
Elmer claims he was trying to expose a widespread system of tax evasion by rich businesspeople and politicians.