"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.
"We'll have to see whether something really happens," a spokesman for the opposition's joint High Negotiations Committee (HNC) said.
"If something happens and if the agreement is implemented, then we will see one another very soon in Geneva [to resume suspended peace talks]," he said.
Talks in Geneva were broken off a week ago after just a few days.
The HNC said it could not sit at the table while Russian aircraft supported a massive regime attack on the city of Aleppo that has sent tens of thousands of people fleeing for the Turkish border.
"The real test will be whether all members of the group stick to their commitments in reality," US Secretary of State John Kerry said.
Meanwhile, Russian Foreign Minister Sergei Lavrov said implementing the deal would be "a complicated task," with "too many forces that are involved in military activities."
But Western countries accuse Russia of bombing civilians in Syria as its air force is deployed to support the Assad regime against opposition groups.
Exceptions to the ceasefire for Islamic fundamentalist groups Isis and al-Nusra may allow Russia and Assad to continue their operations against other opposition fighters, as they have claimed to be fighting Islamists while bombing less extremist enemies in the past.
Some media have also reported sightings of Russian ground troops joining the fighting.
And there was no agreement on Thursday on the future of Assad as President of Syria. Western countries insist that he must be removed, while the Russians hope to keep him in place.
"We know the experiences of the past," Steinmeier said, "that's why I'm not talking about a breakthrough today. It will have to come to light in the coming days whether or not it's a breakthrough."
The German Foreign Minister is a veteran of high-stakes talks with Lavrov and Russian President Vladimir Putin.
Over the past two years, Germany has taken a leading role in attempts to defuse the long-running Ukraine crisis – which have often been set back by Russian double-dealing.
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.
"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.