He said that German Embassy staff in Kabul have been able to visit the man.
The ministry did not give further details.
The weekly Der Spiegel, which did not cite sources, reported that the man - whom it identified as Gholam Ghaus Z., 41 - had traveled to Kabul to visit relatives and was arrested as he tried to buy a razor at a U.S. military supermarket.
U.S. military officials at the Bagram base in Afghanistan declined to answer questions about the detainee, referring queries to the International Committee of the Red Cross, the U.S. Department of Defense and the Afghan Interior Ministry.
ICRC spokeswoman Graziella Leite Piccolo and Afghan Interior Ministry spokesman Zemeri Bashary did not immediately have any information on the German detainee.