"Those in charge recognised that non-diplomatic activities are being undertaken and announced that this diplomat should leave the soil of the Islamic Republic of Iran," Hosseini told a news conference.
Hosseini gave no further details and did not make clear if it was the diplomat who was responsible for the "non-diplomatic activities".
The German embassy in Tehran referred all queries to the foreign ministry in Berlin.
The Hamburg-based weekly Der Spiegel had reported in December that an Iranian official was forced to leave Germany last summer after he tried to acquire components for Tehran's disputed nuclear programme.
Der Spiegel said the Iranian diplomat was expelled after contacting a firm in Bavaria to buy a systems control component which would be essential in the enrichment of uranium.
Iran denies it wants a nuclear bomb and says its atomic work is aimed at boosting civilian power generation.
Associated Press meanwehile quoted Hosseini as saying that Iran has no immediate plans to normalise relations with the United States.