"Today there was a deal that we will immediately begin a significant reduction in violence," Steinmeier told journalists after talks between the US, Russia and other interested nations that continued into the early hours.
"That should flow into an end to battle activities within a week."
The German Foreign Ministry's Twitter account spoke of "new hope" and "real perspectives" for the people of Syria.
The deal struck on Thursday night may open the way for a ceasefire between troops loyal to Syrian President Bashar al-Assad, who is supported by Russia, and opposition fighters.
But attacks on terror group Islamic State would be allowed to continue, Steinmeier added.
The agreement also includes provisions for continued attacks on Al Nusra, an Islamist group affiliated with al Qaeda.
Opening for humanitarians
If the ceasefire does come to pass against all the odds, local powers including Iran and Saudi Arabia have promised to begin delivering aid to the people of Syria.
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"Everyone was agreed about the urgency for humanitarian aid," Kerry said.
A new task force at the United Nations in Geneva will be put in charge of organizing access for aid convoys, Steinmeier said.
The task is huge after five years of civil war in which more than 250,000 people have lost their lives and millions have been driven to flee their homes, either within Syria's borders, to neighbouring countries, or to Europe.
Many parts of the country are completely cut off from outside help.