"We are watching the result in Italy with concern," Steinmeier, who is visiting Greece, told Greek Prime Minister Alexis Tsipras in televised remarks.
"I think Renzi did the right thing, the necessary thing, but voters did not support him," he said through a translator.
"This is not of course a state crisis, but it's a government crisis that needs to be resolved... It's not a positive message to Europe at a difficult time," he added.
Opposition parties had denounced the proposed amendments to the constitution as dangerous for democracy because they would have removed important checks and balances on executive power.
Interior ministry projections suggested the No camp, led by the populist Five Star Movement, had been backed by 59.5 per cent of those who voted.