According to Swiss newspaper SonntagsZeitung, the suspect, who allegedly acted alone and was subsequently arrested, is a former bank employee currently employed in Zurich.
The newspaper added that suspect had sold the data for an undisclosed fee to German tax investigators in the German state of North Rhine Westphalia.
In 2011, German authorities launched a concerted effort to gain the details of German residents and in some instances pay bankers to acquire sensitive client information.
Julius Baer CEO Boris Collardi told SonntagsZeitung that the data pertained to clients who had been using bank secrecy to avoid paying taxes in Germany.
"According to what we know, this is the case," he said.
He added: "As part of our stepped-up controls and a thorough internal investigation we recently discovered a case of data misuse and were able to identify the alleged thief."
The theft is the latest in a line of incidents that have seen tensions rise between Switzerland and Germany.
On a number of occasions, German tax officials have purchased stolen data regarding German clients suspected of tax evasion.
The two countries have made a deal to end the dispute and impose a retroactive levy on declared funds while maintaining secrecy.
Last year Julius Baer agreed to pay German tax authorities EUR50m ($62.6m) to end investigations against the bank, and employees, regarding undeclared assets of German taxpayers.
Source: Private Banker International