The July 2015 agreement officially entered into force after key United Nations confirmation that Tehran has shrunk its atomic programme, triggering the lifting of painful international sanctions on the Islamic republic.
"The Iranian nuclear programme will now for many years be subjected to strict technical restrictions and close monitoring," said Steinmeier, whose country was among the six world powers that negotiated the landmark deal with Tehran.
The diplomatic victory allows for the hope that "we may yet succeed in defusing other urgent crises and conflicts in the region, especially regarding the civil war in Syria," he said in a statement reported by AFP.
"It won't happen overnight and it won't be easy, but neither were the nuclear negotiations with Iran."