The federal prosecutor general confirmed that a 31-year-old German was arrested Wednesday on suspicion of acting for a foreign intelligence service, without specifying which one.
"At the present time we are issuing no further information on the proceedings," a spokeswoman for the federal prosecutor's office said.
But several German media outlets said the suspect was working for a US intelligence agency.
German government spokesman Steffen Seibert said Berlin would wait for the police investigation before reacting but added that spying for a foreign intelligence service was not something "we take lightly".
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Chancellor Angela Merkel was informed of the case yesterday, he said, declining to say whether she had discussed the matter with US President Barack Obama in a telephone call the same day focused on Ukraine.
According to public broadcaster NDR, the man was arrested on initial suspicion of seeking contact with Russian secret services and, in questioning, apparently admitted having handed information to a US agency.
Investigators did not rule out that the suspect had given false information, NDR said.
Last month federal prosecutors also said they had opened a criminal investigation into alleged illegal US snooping on Merkel's mobile phone.
Germans were outraged by revelations last year that the NSA allegedly eavesdropped on Merkel's conversations, as well as about wider US surveillance programmes of Internet and phone communications.
The revelations strained ties between Washington and Germany, a key European ally, which both countries' leaders have been at pains to repair.