"The German government welcomes the signature ... It is naturally important for us that the other conditions be fulfilled" on follow-up inspections and providing information, said foreign ministry spokesman Walter Lindner.
Iranian ambassador Ali Akbar Salehi signed the additional protocol on Thursday at the Vienna headquarters of the UN nuclear watchdog, the International Atomic Energy Agency (IAEA).
The move capped months of intense diplomacy to pressure Tehran into placing its nuclear facilities under greater international supervision and prove that it is not seeking to develop atomic weapons.
Iran had long resisted signing the additional protocol but made an about-face under intense diplomatic pressure in October when the foreign ministers of EU states Germany, Britain and France visited the country.